Add pry, pry-byebug as development dependencies

Basically every time I start working on this library, I go add these two to the Gemfile so that I can step through code and debug things. I think @RST-J uses byebug directly, and this should support that workflow too.

I've added a platform guard on pry-byebug, because it only works on MRI >= 2.0. I've tested this locally on rbx, and travis should hopefully catch any issues with 1.8 and jruby.