update opam
update opam

Summary: upgrades to base 0.12. lwt 4.5.0 on windows was released at some point so we can remove our old hack for that.

Reviewed By: dsainati1

Differential Revision: D25097806

fbshipit-source-id: 68b3d4c98c76095efe056220c7512c63a8f326c5

mroch/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow