Replacing 'pry-nav' with 'pry-byebug'

I've replaced the gem [pry-nav](https://github.com/nixme/pry-nav) with [pry-byebug](https://github.com/deivid-rodriguez/pry-byebug), as the README of `pry-nav` gem is [suggesting](https://github.com/nixme/pry-nav#using-mri-192-switch-to-pry-debugger) to use [pry-debugger](https://github.com/nixme/pry-debugger), but the creator of `pry-debugger` is [suggesting](https://github.com/cldwalker/debugger/issues/125#issuecomment-43358020) to use `pry-byebug`.

Also, the last commits of `pry-nav` and `pry-debugger` where in November, 2013.