Upgrade Rails from 3.2.15 to 3.2.16

Need to specify a version of Tilt otherwise Bundler errors with the untrue:

Bundler could not find compatible versions for gem "tilt":
  In Gemfile:
    sass-rails (= 3.2.5) ruby depends on
      tilt (~> 1.3) ruby

    brakeman (= 2.1.1) ruby depends on
      tilt (2.0.0)
Merge pull request #250 from alphagov/rails-3-2-16

Upgrade Rails from 3.2.15 to 3.2.16