Don't stop if database.sql.gz already exists
The existing behavior of the backups is to overwrite whatever data
was still there in the scratch directories. This broke when we added
a 'gzip' step because 'gzip database.sql' will fail if 'database.sql.gz'
already exists. Doing 'rm -f database.sql.gz' before the 'gzip'
avoids this failure.
See merge request !1011
