"Fix hipchat handler for sensu 0.12.3"

If I use clean hipchat handler I has error:
```
"output":"/etc/sensu/handlers/hipchat.rb:15:in `handle': undefined method `[]' for nil:NilClass (NoMethodError)\n"}
```
It was because settings not contain 'hipchat' options, only ["handlers"]["hipchat"]
just quick fix. I am newby in sensu, and sorry if I miss something. That fix work for me.