Fix duplicate logging for worker process

Without this patch the DJ worker process logs messages twice using
slightly different message formats.  For example:

    2013-01-26T23:39:14+00:00 app[worker.1]: 2013-01-26T23:39:14+0000: [Worker(host:c016b601-6bd9-4e12-a78f-ef971b30f712 pid:2)] PuppetLabs::PullRequestJob completed after 2.0013
    2013-01-26T23:39:14+00:00 app[worker.1]: 2013-01-26T23:39:14+0000: [Worker(host:c016b601-6bd9-4e12-a78f-ef971b30f712 pid:2)] 1 jobs processed at 0.4225 j/s, 0 failed ...
    2013-01-26T23:39:17+00:00 app[worker.1]: 2013-01-26T23:39:17+0000: [Worker(host:c016b601-6bd9-4e12-a78f-ef971b30f712 pid:2)] Exiting...
    2013-01-26T23:39:19+00:00 app[worker.1]: [Worker(host:c016b601-6bd9-4e12-a78f-ef971b30f712 pid:2)] PuppetLabs::PullRequestJob completed after 2.0013
    2013-01-26T23:39:19+00:00 app[worker.1]: [Worker(host:c016b601-6bd9-4e12-a78f-ef971b30f712 pid:2)] 1 jobs processed at 0.4225 j/s, 0 failed ...
    2013-01-26T23:39:19+00:00 app[worker.1]: [Worker(host:c016b601-6bd9-4e12-a78f-ef971b30f712 pid:2)] Exiting...

This patch addresses the problem by only turning on Delayed Job logging through
the `enable :logging` configuration option.