Added mozart-empty as a submodule, updated Rake tasks and versions

* Added github.com/bigcommerce/mozart-empty as a git submodule
* Added Rake task to clean existing JavaScript files from vendor path
* Chained the above to the :release Rake task
* Updated versions of assets and gem
* Updated Mozart version to 0.1.8
Merge pull request #2 from sgerrand/update-to-v0.1.8

Added mozart-empty as a submodule, updated Rake tasks and versions