Updated YARA rules do not re-alert

All YARA matches are saved to DynamoDB, but alerts are only sent to SNS if the YARA rule name has not matched before on the given binary. There are two problems with this:

1. Rules which are renamed or reorganized will re-trigger alerts
2. Rules whose content changes (e.g. a different rule condition) will *not* re-trigger an alert

Instead of looking up based on the rule name, there should be a comparison against some kind of hash of the YARA rule contents