Change debugger to byebug, for ruby2 compatibility.

A fresh dev install in ruby 2.0 or 2.1 fails when trying to install
debugger dependencies. Debugger is not recommended for ruby 2.x
(https://github.com/cldwalker/debugger/issues/125)
Merge pull request #11 from simonalpha/dev_deps_update

Change debugger to byebug, for ruby2 compatibility.