switching to Ruby 2.1.2

- 2.1.2 is recommended on Heroku:
  https://devcenter.heroku.com/changelog-items/468

- `debugger` gem does not build on 2.1.2; switching to `byebug` 
   `bye bug` is the recommended debugger for current Ruby + Rails (see https://github.com/cldwalker/debugger/issues/125#issuecomment-44028274 and https://github.com/rails/rails/issues/14533)

- The following commands may help in switching your local version (assuming you use rbenv):

	rbenv install 2.1.2
	rbenv local 2.1.2
	gem install bundler
	bundle