Fix highlighting of deleted lines in diffs.

Resolves internal 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.

See merge request !1267