Revert "Add pry-byebug for debugging"

This reverts commit 3fc2437b2684bc2aa970773f3edc7b877e47868c.

Since pry-byebug is only compatible with Ruby 2, the checks on Travis
for Ruby 1.9.3 break because it can't run `bundle install` properly.

Since I only used pry-byebug for figuring out one small thing and
haven't needed it before, I'll remove it for the time being.
If I'll need it again, I'll figure out a better solution.