Specify sass-rails version in Gemfile

Fixes https://github.com/thoughtbot/suspenders/pull/325

Bundler could not find compatible versions for gem "sprockets":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      sprockets (~> 2.0.0) ruby

    rails (~> 4.1.0) ruby depends on
      sprockets-rails (~> 2.0) ruby depends on
        sprockets (2.12.1)

Bundler could not find compatible versions for gem "sprockets-rails":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      sprockets-rails (~> 2.0.0) ruby

    rails (~> 4.1.0) ruby depends on
      sprockets-rails (2.1.3)

Bundler could not find compatible versions for gem "railties":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      railties (~> 3.1.0) ruby

    rails (~> 4.1.0) ruby depends on
      railties (4.1.0)