Attempting to appease Travis

    Bundler could not find compatible versions for gem "sprockets":
      In snapshot (Gemfile.lock):
        sprockets (2.12.1)
      In Gemfile:
        sass-rails (~> 4.0.2) ruby depends on
          sprockets (<= 2.11.0, ~> 2.8) ruby

Help from:
https://github.com/sstephenson/sprockets/issues/540