Fix #on_open spec

Since we don't have port 2195 open, EM::Client#connection_completed never
fires.  This change opens a socket on the APNS port so the test can actually
complete end-to-end.
Replace explicit Yajl dependency with MultiJson

@lexer had a pull request open for this for two years. Boo on us.
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
Not sure what this comment means
Merge pull request #11 from groupme/make-truncate-method-less-dumb

Make truncate_alert! method handle symbol keys
Change push notification size limit from 256 to 2048
Merge pull request #16 from dfried/increase_push_notification_size

Change push notification size limit from 256 to 2048