Log Message:
Update ruby-sprockets to 3.5.2.

**3.5.2** (December 8, 2015)

* Fix JRuby bug with concurrent-ruby.
* Fix disabling gzip generation in cached environments.

**3.5.1** (December 5, 2015)

* Fix gzip asset generation for assets already on disk.

**3.5.0** (December 3, 2015)

* Reintroduce Gzip file generation for non-binary assets.

**3.4.1** (November 25, 2015)

* PathUtils::Entries will no longer error on an empty directory.

**3.4.0** (October 5, 2015)

* Expose method to override the sass cache in the SassProcessor.