Gemfile: get rails from 3.2-stable branch

There were again multiple security issues that were fixed upstream:

  http://weblog.rubyonrails.org/2014/7/2/Rails_3_2_19_4_0_7_and_4_1_3_have_been_release

In order to keep safe in the future, let's get our sources from the 3-2-stable branch of the official rails repository, where such fixes end up even before a release is out.