Remove explicit setting of dispatcher

Older versions of `newrelic_rpm` required setting the dispatcher for the agent to report. This hasn't been the case for most of the last year, though.

Setting the dispatcher on `manual_start` wouldn't be a problem except DelayedJob (and potentially Resque) are 1) run through rake and 2) `newrelic_rpm` relies on the background framework being detected as dispatcher to function properly.

Explicitly setting the `:dispatcher => :rake` short-cuts that detection, and things break. Removing that dispatcher will only make processes on the environment page not list 'rake' preceding the hostname, which is a bummer, but
better than DJ clients missing instrumentation.

Thanks for much for maintaining this extension to New Relic! :+1: :sparkles: