Drop support for GitLab <= 6.6

modernizr gem has been removed from rubygems.org repository, and GitLab
versions lower than 6.7 currently cannot be installed fully
automatically (fix for removal has been commited to 6-7-stable branch).
Since ginas supports GitLab 6.7 and 6.8 already, which allows for
upgrade tests, I'm removing support for older versions.

More information: https://github.com/gitlabhq/gitlabhq/issues/6687
Add PostgreSQL database support in 'gitlab' role

- 'postgresql' role is currently not very secure, I recommend using
  MySQL in production environment;
- upgrade procedure works with PostgreSQL;
- upgrades from MySQL to PostgreSQL or vice versa haven't been tested
  and are not recommended;
Merge pull request #59 from drybjed/role/gitlab

Updates in GitLab role