Fix for keepalive

Here is the fix for keepalive. Note that I had to remove the "connection" part from the example https://github.com/node-xmpp/node-xmpp#keepalives.

I booted hubot and everything went well so I guess there are not other regression :-/

I also changed the "main" in package.json so it can use the .js if the .coffee file was transpiled. This is usefull to run hubot in debug in IntelliJ and should still load the .coffee when the .js is not available.