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)