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.

lib/backup/database.rb

@@ -25,6 +25,7 @@ module Backup
       abort 'Backup failed' unless success
       $progress.print 'Compressing database ... '
+      FileUtils.rm_f db_file_name_gz
       success = system('gzip', db_file_name)
       report_success(success)
       abort 'Backup failed: compress error' unless success