Gevent installs incompatible greenlets version

[MJJoyce]

Credit to @nttoole for tracking this down.

Our Core and DSN builds started breaking out of the blue ~9 days ago. I didn't see anything that really jumped out when digging through the build logs beyond a hint that gevent.greenlet size changed and that might indicate a binary incompatibility.

It seems like greenlets pushed 0.4.17 9 days ago and this had some backwards incompatible changes. Judging from this gevent issue they don't pin to a specific greenlets version in their releases. Should be able to fix this by pinning to greenlets==0.4.16