ruby 2.0.0 -> 2.1.3

A couple of people at this weekend's NYC RailsBridge event who were running MacOS Yosemite were having issues with ruby 2.0.0 segfaulting.  Upgrading to 2.1.3 fixes the issue, and for the purposes of the RailsBridge tutorials, 2.1.3 should be a drop-in replacement for 2.0.0.