Make the Gemfile version for Rails squiggly to fix a Ruby 2.0 bug in Sprockets

Turns out the bug I was running into was a change in how respond_to? works in Ruby 2.0! 

https://github.com/sstephenson/sprockets/commit/6ac4248f5817a430a1e620ab214501a641cbdc5e

I'm adding a dependency on byebug too, because it's super useful for debugging. But let me know if that's TOO MUCH.