Do not shutdown Agent after performing if Resque forks are disabled.

If `ENV['FORK_PER_JOB'] = 'false'`, Resque does not fork to perform each job. The existing behavior ensures that the Agent is shut down after performing each job. This causes Resque workers to report data for one job after starting up, and then ignore the rest until the workers are restarted again.

This change just adds a check to make sure that the Agent is not shut down in this situation, just like when `Resque.inline` is enabled.

This started out as an issue on the community forums:
https://discuss.newrelic.com/t/new-relic-agent-not-reporting-from-resque-worker/7800