The rubocop only runs in the CI this way a contributor probably will see a
rubocop offense only in the CI.
Running the rubocop in the default rake task we have more chance that a offense
be get in the local machine.
gemspec requires "rails" and "activemodel, ">= 4.0", so testing
on 3.2 will always fail to resolve dependencies.
adds and defaults to Rails 4.2 when RAILS_VERSION is not specified
It is impossible to run our tests on both Rails 3.2 and Rails 4:
1. Rails 3.2 relies on test/unit, which is MiniTest 4.
2. Rails 4 relies on MiniTest 5.
3. MiniTest 5 is not compatible with test/unit.
😢😢😢
