Minor tweak to install docs for postgres.

Installing gems with postgres should be "--without mysql" instead of
"postgres".  Also, database.yml example is ".postgresql" instead of
".postgres".
Merge pull request #209 from tom5760/minor-install-doc-tweak-postgres

Minor tweak to install docs for postgres.
Introduced $APP_USER var for running the service

...as that $APP_USER

Solution for gitlabhq/gitlab-ci#186