upgrade sprockets
cache digests need an initializer to work with haml

https://github.com/rails/cache_digests/pull/46
proper tracking of cache invalidation for menus

people and group menu used the same cache key before.

Also cache for _top_menus depends on _groups_menu and _people_menu templates
Merge pull request #92 from azul/bugfix/dropdown-menu-cache-fix

Bugfix/dropdown menu cache fix