not compatible with gevent 1.5.0 released on April 10, 2020

[brainfull]

need to use gevent 1.4.0 because of the following

[INFO] redis: Connecting to Redis at service-central-station...
Traceback (most recent call last):
File "/usr/local/bin/mrq-worker", line 10, in
sys.exit(main())
File "/usr/local/lib/python3.7/site-packages/mrq/bin/mrq_worker.py", line 53, in main
w.work()
File "/usr/local/lib/python3.7/site-packages/mrq/worker.py", line 453, in work
self.work_init()
File "/usr/local/lib/python3.7/site-packages/mrq/worker.py", line 491, in work_init
self.install_signal_handlers()
File "/usr/local/lib/python3.7/site-packages/mrq/processes.py", line 40, in install_signal_handlers
gevent.signal(signal.SIGINT, request_shutdown_graceful)
TypeError: 'module' object is not callable