Log Message:
Update ruby-sprockets to 3.7.0.

** 3.7.0** (July 21, 2016)

* Deprecated interfaces now emit deprecation warnings #345

**3.6.3** (July 1, 2016)

* Faster asset lookup in large directories #336
* Faster PathUtils.match_path_extname \ 
https://github.com/rails/sprockets/commit/697269cf81e5261fdd7072e32bd489403027fd7e
* Fixed uglifier comment stripping #326
* Error messages now show load path info #313

**3.6.2** (June 21, 2016)

* More performance improvements.

**3.6.1** (June 17, 2016)

* Some performance improvements.

**3.6.0** (April 6, 2016)

* Add `Manifest#find_sources` to return the source of the compiled assets.
* Fix the list of compressable mime types.
* Improve performance of the `FileStore` cache.