WIP: Add rb-readline to the development gems

Hopefully fixes #81 (closed).

This change adds Readline support to Ruby on macOS as described in https://github.com/guard/guard/wiki/Add-Readline-support-to-Ruby-on-Mac-OS-X.

If you are on Mac OS X and have problems with either Guard not reacting to file changes or Pry behaving strangely, then you probably suffer under a Ruby build that uses libedit instead of readline.