Fix unsafe Resque::Scheduler shutdown in Ruby 2.0

This makes shutdown compatible with Ruby 2.0 by not calling
release_master_locks! in the Resque::Scheduler signal handler.
Make things a little less racey

This fixes raciness concerns raised (hrhr) by @fbogsany. MRI should be
fine, but there's still an element of raciness in other Rubies due to
the inability to use mutexes in signal handlers (or without any proper
memory barriers).
Merge branch 'fix_shutdown_when_sleeping'

Conflicts:
	lib/resque/scheduler.rb
	resque-scheduler.gemspec