Change reference to 'drain' event to 'completed'

https://github.com/argon/node-apn/blob/master/lib/connection.js#L311 seems to indicate the doc uses the wrong event name here and confirmed by a problem I had today where a 'drain' event never was emitted (but 'completed' was).
Merge pull request #209 from nathanwdavis/master

Change reference to 'drain' event in docs to 'completed'