www/gitlab-ce: fix rubygem-rdoc problem

The upgrade of rubygem-rdoc to 6.4 in commit
1a6659dcb611aab06e7e9ba3adee8c81ce3f9d93
broke gitlab.
Use version rdoc 6.4 does not work due to segfaults, please see
commit log of devel/rubygem-rdoc63 for more details.

PR: 260679
Reported by: lysfjord.daniel@smokepit.net