Bump concurrent-ruby from 1.2.3 to 1.3.4

Bumps [concurrent-ruby](https://github.com/ruby-concurrency/concurrent-ruby) from 1.2.3 to 1.3.4.
- [Release notes](https://github.com/ruby-concurrency/concurrent-ruby/releases)
- [Changelog](https://github.com/ruby-concurrency/concurrent-ruby/blob/master/CHANGELOG.md)
- [Commits](ruby-concurrency/concurrent-ruby@v1.2.3...v1.3.4)

---
updated-dependencies:
- dependency-name: concurrent-ruby
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -45,7 +45,7 @@ GEM
     codecov (0.6.0)
       simplecov (>= 0.15, < 0.22)
     coderay (1.1.3)
-    concurrent-ruby (1.2.3)
+    concurrent-ruby (1.3.4)
     connection_pool (2.4.1)
     diff-lcs (1.5.0)
     docile (1.4.0)