use debugger or byebug dependent on ruby version

debugger doesn't support ruby >= 2  (http://rubydoc.info/gems/debugger/1.6.8/frames)
the (not huge) problem with putting conditionals in the Gemfile is that the the lockfile won't work in both conditions.

I'll go ahead and merge this in, but I think post 1.13 we can probably start assuming developers have ruby 2+.  (as soon as we upgrade to rails 4, we'll require at least ruby 1.9.3 anyway)