Update example to include explicit mapper for Delayed::Job

Changed as of https://github.com/meskyanichi/hirefire-resource/commit/895be2dfcb59e017e352216f84013eccaf23e204#diff-2502e98b7eb8903d6012429ca63afcf4
Merge pull request #15 from justincampbell/patch-1

Update example to include explicit mapper for Delayed::Job.