Add Dependabot to track dependencies

[iMacTia]

Issue description

As pointed out here, we can make contributors life easier by tracking more proactively updates to our dependencies.
We should introduce Dependabot as it was recently acquired by GitHub, it's free and should be easy to setup.

Dependencies to check

Here is a list of dependencies we might want to ask Dependabot to check:

• Rubocop
• Rspec
• Simplecov
• Webmock

I'm unsure about adapters, as the plan is to move them out, but we may be testing against some pretty old versions...