Empty the database during Postgres backup restore

The expected behavior during a GitLab backup restore is to overwrite
existing database data. This works for MySQL because the output of
mysqldump contains 'DROP TABLE IF EXISTS' statements. pg_dump on the
other hand assumes that one will restore into an empty database. When
this is not the case, during the restore with psql some of the data will
be skipped if existing data is 'in the way'. By first invoking `rake
db:schema:load` during a Postgres GitLab backup restore, we make sure
that all important data is correctly restored.
 Merge branch 'pg_force_restore' into 'master'

Pg Force Restore
Use Gitlab::VERSION to version backups

Previous to this commit, backups were tied to the git revision (SHA1) of
the app at the time the backup:create command was invoked. If the SHA1
at the time of restore was different, the script would refuse to restore
the backup.  This commit loosens this tie so that the backup script only
complains if the value of the Gitlab::VERSION constant is different
between the time of backup and the time of restore.
 Merge branch 'backup_version' into 'master'

Less strict backup versions
Merge branch 'master' of github.com:gitlabhq/gitlabhq