Support added for Rails 4.2.

[zhenyuchen]

Rails 4.2.0.beta2 has updated rack to 1.6.0.beta, so rack need change the version to >= 1.5.2.

Bundler could not find compatible versions for gem "rack":
  In Gemfile:
    rack-tracker (~> 0.2.3) ruby depends on
      rack (~> 1.5.2) ruby

    rails (= 4.2.0.beta2) ruby depends on
      actionpack (= 4.2.0.beta2) ruby depends on
        rack (1.6.0.beta)