Switch to byebug gem for debugging

* `byebug` has a better support for Ruby 2.0 than `debugger`
* `debugger` statement can still be used (with all the familiar commands)
Revert "Use Linguist to determine file language"

* Deploying to Heroku turned out to be huge pain because of libicu.

This reverts commit 4d0ec94830d3ba0cb9c665cf61c8c10a0f2bdd96.
Refactor ModifiedFile to CommitFile

Generalize the name to handle files that may not have been modified, like
`.hound.yml`.
Support exclusion of files from the config

Check file exculsion via Rubocop's `Config#file_to_exclude?` method.
Prefer double quotes everywhere

Updated as per new desicion in our styleguide
* Revert to original purple