Support for ruby 2.1.2

Installation of jazz_hands is currently failing on ruby 2.1.2 because of it's dependencies - particularly `pry-debugger` which is currently broken for 2.1.2, because of it's dependency `debugger`. Even though `debugger` works with rubies 2.0 up to 2.1.1, it's not supposed to (more info here: https://github.com/cldwalker/debugger/issues/125#issuecomment-43353446 and here https://github.com/cldwalker/debugger/issues/125#issuecomment-43358018). As ruby 2 is used more and more frequently this change might be beneficial.