tests

Pass in `pidfile: nil` if `tmp/pids/` doesn't exist #157

Job	Run time
rspec (3.0)	40s
active_record_with_lock_adapter_rspec (3.0)	58s
smoke_test (3.0, 2.2.5)	34s
rspec (3.1)	41s
rubocop	10s
active_record_with_lock_adapter_rspec (3.1)	39s
smoke_test (3.0, 3.1)	41s
rspec (3.2)	38s
active_record_with_lock_adapter_rspec (3.2)	53s
smoke_test (3.1, 2.2.5)	43s
active_record_with_lock_adapter_rspec (3.3)	51s
rspec (3.3)	1m 4s
smoke_test (3.1, 3.1)	43s
smoke_test (3.2, 2.2.5)	1m 10s
smoke_test (3.2, 3.1)	38s
smoke_test (3.3, 2.2.5)	42s
smoke_test (3.3, 3.1)	59s
	12m 44s

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