Change resque-rollbar source

https://github.com/dimko/resque-rollbar is the source for the rubygems version of resque-rollbar.  It is now functionally equivalent to CrowdFlower/resque-rollbar after https://github.com/dimko/resque-rollbar/pull/7.  Given that, CrowdFlower is going to deprecate their version and point it to @dimko's.