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.