Stop using fork of locust to unblock loadtests
Stop using fork of locust to unblock loadtests

Sometime in the last 30 days, using our fork of locust broke locust's ability to drive significant load. I observed the following behavior:

* Request 200 users at 10 users/s
* Locust begins spawning users at the requested rate, req/s increases linearly with users
* Locust user spawn rate and req/s start decreasing. Req/s levels off to a test-specific constant (varies between tests but not between runs of tests)
* Locust continues to spawn users albeit at a vastly lower rate. Req/s decrease slowly

Switching to the standard locust distribution solves the issue.
edx/edx-load-tests,edx/edx-load-tests,edx/edx-load-tests,edx/edx-load-tests