Fix #has_asset_pipeline? to work with Rails 4

In Rails 4, the asset pipeline is enabled by default, and
config.assets.enabled is nil.
Fix dependency of i18n/filtered on all locale files

Not sure why this works and the previous version didn't. Copied this
from PikachuEXE's fork of i18n-js. One calls #register_preprocessor on
Rails.application.assets, and the other calls it on Sprockets.