Switched to coq 8.4 patched

The eta expansion stuff is gone, because coq 8.4 has eta conversion.

We are using the following versions:

DanGryason/
    rezk_completion 98e2ee14840021ce1a5684d22b2a0719278e30b8
    Foundations2    9e4e62e7964189217baec0e602f153e9a71ca8a0
    coq             721167c315dba3d57e83cc4a2bc1cdd7b8bef62e