Switch to using mono_logger instead of stdlib logger

as this is what Resque uses and it *should* help with odd bugs that crop
up with Ruby 2 and traps.
Merge pull request #374 from meatballhat/switch-to-mono-logger

Switch to using mono_logger instead of stdlib logger
Preparing for v2.5.5
Merge pull request #379 from meatballhat/v2.5.5-prep

Preparing for v2.5.5
Merge remote-tracking branch 'upstream/v2.x-stable' into merging-v2.x-stable

Conflicts:
	README.md
	lib/resque_scheduler/logger_builder.rb
	lib/resque_scheduler/version.rb
	resque-scheduler.gemspec
	test/scheduler_setup_test.rb
Merge pull request #387 from meatballhat/merging-v2.x-stable

Merging v2.x stable