Fix railtie, loading

v0.0.6 has a couple of bug introduced by my reorganization and addition of the railtie:

1. The `app` yielded to to the initializer in the railtie doesn't contain the Rails cache. It's only available from the `Rails` constant itself.
2. Loading the railtie before the ActiveRemote::Cached module doesn't fire the ActiveSupport::Concern hooks, which means none of the class methods are extended. Loading the railtie after the module is interpreted fixes the problem.

I've verified that this is now working as expected.