assets:precompile: do not reset the whole cache.

By default, on rake assets:precompile, the whole cache is deleted. But to re-create the cache takes several hours. We cannot afford this on each deployment since we deploy several times a day.

Therefore, we override the precompile behaviour to clear only tmp/cache/assets.

References:
* http://www.reinteractive.net/posts/116-12-tips-for-the-rails-asset-pipeline
* https://github.com/rails/sprockets-rails
* https://github.com/rails/rails/blob/v3.2.3/actionpack/lib/sprockets/assets.rake\#L69

Trello: https://trello.com/c/gqhbW3qq/646-performance-optimierung-von-gruppen