Nonces: add ORDER BY when deleting nonces

It seems it should avoid the following error:
```
mysqld: 140131 0:07:13 [Warning] Statement may not be safe to log in statement format. Statement: DELETE FROMwp_optionsWHEREoption_nameLIKE 'jetpack\\_nonce\\_%' AND CAST(option_valueAS UNSIGNED ) < 1391119633 LIMIT 100
```

This seems to come from an issue with mySQL:
* https://dev.mysql.com/doc/refman/5.1/en/replication-features-limit.html

Reported here:
* http://wordpress.org/support/topic/mysql-warning-statement-may-not-be-safe-to-log-in-statement-format?replies=1

I'd be happy to have a second set of eyes on this as mySQL is not my strong suit. :)