Move to Ruby 2.3.6 and Bundler 1.15.2

[leehuffman]

This moves the application to the latest Ruby 2.3.x version, 2.3.6, and bumps Bundler to 1.15.2, the current version that Heroku officially supports.

https://devcenter.heroku.com/articles/ruby-support#supported-runtimes
https://devcenter.heroku.com/articles/ruby-support#libraries

Successful script/cibuild output:

$ script/cibuild
+ git log -n 1 HEAD
+ cat
commit 1b4c5a821551d3a258f0ba03d29763f3253c0da2
Author: Lee Huffman <huffman@github.com>
Date:   Sat Feb 3 16:05:45 2018 -0800

    Bump Bundler to 1.15.2 in Gemfile.lock
+ ruby -v
ruby 2.3.6p384 (2017-12-14 revision 61254) [x86_64-darwin16]
+ bundle -v
Bundler version 1.15.2
The latest bundler is 1.16.1, but you are currently running 1.15.2.
To update, run `gem install bundler`
+ set +x
Using rake 10.4.2
Using i18n 0.7.0
Using json 1.8.2
Using minitest 5.6.1
Using thread_safe 0.3.5
Using builder 3.2.2
Using erubis 2.7.0
Using mini_portile2 2.1.0
Using pkg-config 1.1.7
Using rack 1.6.4
Using mime-types 2.5
Using arel 6.0.0
Using addressable 2.3.6
Using ast 2.0.0
Using coderay 1.1.0
Using debug_inspector 0.0.2
Using bundler 1.15.2
Using callsite 0.0.11
Using multipart-post 2.0.0
Using yajl-ruby 1.2.0
Using highline 1.6.21
Using net-ssh 2.9.1
Using safe_yaml 1.0.4
Using diff-lcs 1.2.5
Using dotenv 0.9.0
Using dpl 1.5.7
Using multi_xml 0.5.5
Using multi_json 1.12.0
Using thor 0.19.1
Using kgio 2.9.2
Using method_source 0.8.2
Using mono_logger 1.1.0
Using pg 0.17.1
Using posix-spawn 0.3.8
Using powerpack 0.0.9
Using slop 3.6.0
Using rails_serve_static_assets 0.0.2
Using rails_stdout_logging 0.0.3
Using rainbow 2.0.0
Using raindrops 0.13.0
Using redis 3.3.0
Using tilt 2.0.2
Using rspec-core 2.14.7
Using rspec-mocks 2.14.5
Using ruby-progressbar 1.5.1
Using simplecov-html 0.7.1
Using slack-notifier 1.0.0
Using sqlite3 1.3.10
Using tzinfo 1.2.2
Using nokogiri 1.6.8
Using rack-test 0.6.3
Using rack-contrib 1.1.0
Using rack-protection 1.5.3
Using sprockets 3.0.3
Using vegas 0.1.11
Using warden 1.2.3
Using mail 2.6.3
Using parser 2.2.2.5
Using better_errors 1.1.0
Using binding_of_caller 0.7.2
Using faraday 0.9.0
Using net-scp 1.2.1
Using net-sftp 2.1.2
Using net-ssh-gateway 1.2.0
Using crack 0.4.2
Using rspec-expectations 2.14.5
Using httparty 0.13.1
Using foreman 0.63.0
Using pry 0.9.12.6
Using rails_12factor 0.0.2
Using unicorn 4.8.2
Using redis-namespace 1.5.2
Using simplecov 0.7.1
Using activesupport 4.2.2
Using loofah 2.0.2
Using aws-sdk 1.51.0
Using sinatra 1.4.7
Using astrolabe 1.3.0
Using faraday_middleware 0.9.1
Using sawyer 0.5.5
Using capistrano 2.9.0
Using webmock 1.17.3
Using flowdock 0.5.0
Using hipchat 1.1.0
Using rails-deprecated_sanitizer 1.0.3
Using globalid 0.3.5
Using activemodel 4.2.2
Using rails-html-sanitizer 1.0.2
Using resque 1.26.0
Using rubocop 0.26.0
Using campfiyah 0.0.6
Using octokit 3.4.0
Using rails-dom-testing 1.0.6
Using activejob 4.2.2
Using activerecord 4.2.2
Using resque-lock-timeout 0.4.4
Using warden-github 1.0.2
Using actionview 4.2.2
Using actionpack 4.2.2
Using actionmailer 4.2.2
Using railties 4.2.2
Using sprockets-rails 2.2.4
Using meta_request 0.2.8
Using rspec-rails 2.14.1
Using warden-github-rails 1.1.0
Using rails 4.2.2
Updating files in vendor/cache
Bundle complete! 29 Gemfile dependencies, 106 gems now installed.
Bundled gems are installed into ./vendor/gems.
The latest bundler is 1.16.1, but you are currently running 1.15.2.
To update, run `gem install bundler`
warning: parser/current is loading parser/ruby22, which recognizes
warning: 2.2-compliant syntax, but you are running 2.3.6.
warning: parser/current is loading parser/ruby22, which recognizes
warning: 2.2-compliant syntax, but you are running 2.3.6.
warning: parser/current is loading parser/ruby22, which recognizes
warning: 2.2-compliant syntax, but you are running 2.3.6.
........DEPRECATION WARNING: `named_routes.helpers` is deprecated, please use `route_defined?(route_name)` to see if a named route was defined. (called from block (2 levels) in <top (required)> at /Users/leehuffman/src/heaven-leehuffman/spec/controllers/concerns/webhook_validations_spec.rb:28)
DEPRECATION WARNING: `named_routes.helpers` is deprecated, please use `route_defined?(route_name)` to see if a named route was defined. (called from block (2 levels) in <top (required)> at /Users/leehuffman/src/heaven-leehuffman/spec/controllers/concerns/webhook_validations_spec.rb:30)
....................................................................

Finished in 0.5663 seconds
76 examples, 0 failures

Randomized with seed 20924

Coverage report generated for RSpec to /Users/leehuffman/src/heaven-leehuffman/coverage. 7881 / 15165 LOC (51.97%) covered.
warning: parser/current is loading parser/ruby22, which recognizes
warning: 2.2-compliant syntax, but you are running 2.3.6.
Inspecting 87 files
.......................................................................................

87 files inspected, no offenses detected