Update rufus-scheduler to 3.0+

rufus-scheduler 3.0+ has a better story for locking the scheduler
(so only one scheduler process can actually enqueue jobs).

The main change is that reloading a schedule doesn't conserve any old
info, and instead replaces the old schedule wholesale with whatever is
found in Redis.

All other changes seem minor:
- replace rufus_scheduler#all_jobs with rufus_scheduler#jobs
- monkeypatch Rufus::Scheduler::Job#params

Nonetheless, rufus-scheduler warns that the scheduler in 3.0 has changed
and could schedule jobs differently from rufus-scheduler 2.0. Buyers
beware.
Add pry to gemspec.

The ultimate debugging tool?
Pass initialize options to Rufus::Scheduler.

Need to pass a lock? Now you can!