Fix alert truncation when using symbols as keys.
Normalize APS keys to strings

We need to access these keys in order to massage the payload so convert each
key to a string on initialization.

* Avoid ActiveSupport dependency and use our own Hash#stringify_keys! shim
* Change specs to be positive to avoid confusing failure messages
* Minor clean-up of Notification initializer
Merge pull request #11 from groupme/make-truncate-method-less-dumb

Make truncate_alert! method handle symbol keys