Starting from GitLab 8.6 pg_trgm extension becomes hard dependency of
gitlab.

    https://gitlab.com/gitlab-org/gitlab-ce/commit/d24ee2a2

The extension can be activated only by db superuser, so gitlab db
migration scripts does not activate it - it has to be done by DB
administrator or is handled by integrating code in omnibus case.

As we already handle DB setup and migrations in unicorn startup script,
as pre-action there, let's activate pg_trgm.