Rails 4.1. See https://github.com/rails/sprockets-rails/pull/84

Fixes issue #176, caused by a change on Rails 4.1.

Rails 4.1 actively verifies that any assets served on development can actually be served on production. This behaviour is controlled by the config setting `app.config.assets.raise_runtime_errors`, which by default is `false` on test and `true` on other environments. See https://github.com/rails/sprockets-rails/pull/84 for discussion.

JS tests don't fulfil this requirement because they are not listed for precompilation. Simply setting `raise_runtime_errors` to `false` would not be a good solution, as users would be back to not noticing when their assets won't work on production.

Instead, this commit solves the problem by adding the test files to the precompilation list.