fix failing tests on Rails 4.0

rails/sprockets-rails#116 was introduced in released version 2.1, causing the
`Sprockets::Rails::Helper` initialization not to be loaded unless
`ActionView::Base` is loaded. This doesn't cause any issues with actual use of
spritely, but does cause issues within our tests when we check the result of
rendered assets programmatically via a Rails console, when `ActionView::Base`
has not been loaded.

We now explicitly call `ActionView::Base` before outputting the rendered asset,
to force app configuration to be loaded appropriately.
start testing separately against Rails 4.2 beta
enable asset digests across all tested versions of Rails

Rails 4.2 turns on asset digests in development