Exclude the gitlab-rails software from the cache
Exclude the gitlab-rails software from the cache

This is one of the last software items, and will almost always need to
be rebuilt anyways.

Excluding this from the cache should keep our cache bundle size a bit
smaller, as this is the software item with the most churn.

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab