jasnow/Practice-Git-with-Rails — pull request #1 by jakswa (2013-05-23 23:28:42+00:00)
Title: specifying ruby version in the gemfile

this is described on the bundler website here:
http://gembundler.com/v1.3/gemfile_ruby.html

services like Heroku use this directive to know what version of ruby to compile your assets in.