Rewrite false positive test

The previous version of rollbar_spec.rb would pass even if `lib/resque/rollbar.rb`'s `before_first_fork` block was empty since it used the default Rollbar configuration which is synchronous.
Reset Rollbar in Resque.before_first_fork

Fixes cases where a `Notifier` may have been created prior to the Rollbar initializer calling `Rollbar.configure`. Previously Resque would continue to use this prematurely created `Notifier` and never use the `Configuration` defined by the initializer. See https://github.com/rollbar/rollbar-gem/issues/183
Update to support only Rollbar > 1.2.x
Merge pull request #8 from jonah-williams/reset_notifier_before_first_fork

Reset notifier before first fork