Upgrade "resque-scheduler" to "3.0.0"

* Upgrade "resque-scheduler" gem
* Reference "resque/scheduler/tasks" instead of "resque_scheduler/tasks"
* Update "README.md" to reflect new requirement
Merge pull request #108 from jzaleski/jwz-upgrade-resque-scheduler-gem

Upgrade "resque-scheduler" to "3.0.0"