Prepare for 0.5.1.0 release

Summary:
We need a release for GHC 8.2.1 compatibility:
https://github.com/facebook/Haxl/issues/70

(Note: this ignores all push blocking failures!)

Reviewed By: simonmar

Differential Revision: D5488485

fbshipit-source-id: 61ae115e7456beb1dd8855d387690e13a1e09bdc