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.