gitlab-backup: time module is required for Time.parse

Solve the problem:

    NoMethodError: undefined method `parse' for Time:Class

--------

kirr: it probaly started to fail due to gitlab codebase upgrade, as eariler gitlab-rake was likely requiring "time" and now does not.

/reviewed-by @kirr
/reviewed-on !6

contrib/gitlab-backup

@@ -302,7 +302,7 @@ backup_restore() {
                         sed -e s'/:backup_created_at: //'`
     # 2015-08-27 11:32:37.201345216 +02:00 -> 1440667957
     backup_created_at=`echo "$backup_created_at" |
-                        gitlab-rake -e 'puts Time.parse(STDIN.read).to_i'`
+                        gitlab-rake -e 'require "time"; puts Time.parse(STDIN.read).to_i'`
 
     # 3. prepare tarball as would be created by gitlab:backup:create
     backup_tar="${backup_created_at}_gitlab_backup.tar"