[BUG]change rule has logic error when the compare_key is boolean

[anjia0532]

I use Heartbeat monitor my service . and alert by change rule.

query_key: monitor
compare_key: up
ignore_null: true

but the Heartbeat's up field is Boolean value 。so...
https://github.com/Yelp/elastalert/blob/master/elastalert/ruletypes.py#L135

...
 def compare(self, event):
        key = hashable(lookup_es_key(event, self.rules['query_key']))
        val = lookup_es_key(event, self.rules['compare_key'])
        if not val and self.rules['ignore_null']:
            return False
...

when the service down

up is false. not val is true and because of the ignore_null is true.

so always return False not run compare code