Fix highlighting of deleted lines in diffs.

Resolves internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2498.

Only needed until https://github.com/jneen/rouge/pull/297 is merged into Rouge and the gem is updated in GitLab.

Forking rouge, releasing gitlab-rouge and using that gem wasn't an option, since gollum-lib has rouge as a dependency, so it would get included anyway, and which code we'd get would depend on the load order.