Allows use of Sprockets manifest path for rjs_manifest.yml

Hi John, I am not using public/assets as the directory for the manifests (I actually remove its contents so to not keep generated assets it in the repo) - rather, I use a custom location for the manifest file - config.assets.manifest.

This pull request makes RequireJS-Rails to first try use a configured path - config.assets.manifest - and otherwise falls back to public/assets.

Thanks, Jürgen

btw - tried to get the tests/specs running but gave up after 5mins - can you pls include a little Snippet in the Readme.md on how to contribute? Thanks!!