minor: fix Gemfile to specify activesupport up to 4.x if using < 1.9.3

activesupport version 4.0.0, released on June 25th 2013, requires Ruby version >= 1.9.3
I've updated the Gemfile to install up to v 4.x for ruby versions < 1.9.3 and the latest stable for anything >= 1.9.3