[JS] Don't return a value in the PhantomJS Driver constructor

JavaScript functions shouldn't return any value if they're being invoked with
the 'new' operator. In this case it has a very nasty side effect: the PhantomJS
driver seems to work just fine, however invoking the quit() method will not kill
the service process and thus stray phantomjs processes will be left floating
around. This can severely impact the running system when hundreds of flows are
being created.