Fix compatibility with rails 4.2

Fixes this bug on upgrading to rails 4.2: 
Bundler could not find compatible versions for gem "activesupport":
  In Gemfile:
    rails (~> 4.2) ruby depends on
      actionview (= 4.2.0) ruby depends on
        activesupport (= 4.2.0) ruby

    rails (~> 4.2) ruby depends on
      actionpack (= 4.2.0) ruby depends on
        rails-dom-testing (>= 1.0.5, ~> 1.0) ruby depends on
          activesupport (4.2.0.beta1)