Fix OCaml 4.01
Fix OCaml 4.01

Summary: `Set.of_list` was introduced in 4.02 -- so our 4.01 builds broke. My bad!

Reviewed By: gabelevi

Differential Revision: D3446187

fbshipit-source-id: 8ce31ec116363222dc00d4d80cf025f601c2940a

gabro/flow,nmote/flow,jgrund/flow,facebook/flow,fletcherw/flow,AprilArcus/flow,gabelevi/flow,ylu1317/flow,cesarandreu/flow,mroch/flow,chenglou/flow,ForbesLindesay/flow,jeffmo/flow,AprilArcus/flow,popham/flow,gabelevi/flow,jamesgpearce/flow,jeffmo/flow,jdelStrother/flow,MichaelDeBoey/flow,jeffmo/flow,fletcherw/flow,samwgoldman/flow,TiddoLangerak/flow,MichaelDeBoey/flow,facebook/flow,fletcherw/flow,jamesgpearce/flow,facebook/flow,jdelStrother/flow,gabelevi/flow,claudiopro/flow,samwgoldman/flow,cesarandreu/flow,ForbesLindesay/flow,JonathanUsername/flow,AprilArcus/flow,AprilArcus/flow,ylu1317/flow,JonathanUsername/flow,TiddoLangerak/flow,AgentME/flow,fletcherw/flow,jdelStrother/flow,TiddoLangerak/flow,AgentME/flow,jdelStrother/flow,mroch/flow,facebook/flow,gabelevi/flow,nmote/flow,chenglou/flow,claudiopro/flow,chenglou/flow,mroch/flow,fletcherw/flow,ForbesLindesay/flow,jeffmo/flow,jamesgpearce/flow,nmote/flow,jgrund/flow,JonathanUsername/flow,mroch/flow,samwgoldman/flow,AgentME/flow,cesarandreu/flow,JonathanUsername/flow,gabro/flow,JonathanUsername/flow,mroch/flow,gabelevi/flow,cesarandreu/flow,jgrund/flow,gabro/flow,jdelStrother/flow,jamesgpearce/flow,gabro/flow,chenglou/flow,ylu1317/flow,popham/flow,nmote/flow,jdelStrother/flow,claudiopro/flow,MichaelDeBoey/flow,gabelevi/flow,gabro/flow,nmote/flow,claudiopro/flow,nmote/flow,popham/flow,samwgoldman/flow,popham/flow,jgrund/flow,facebook/flow,jamesgpearce/flow,ForbesLindesay/flow,jgrund/flow,chenglou/flow,MichaelDeBoey/flow,TiddoLangerak/flow,ylu1317/flow,gabro/flow,chenglou/flow,samwgoldman/flow,AgentME/flow,ForbesLindesay/flow,jgrund/flow,AprilArcus/flow,samwgoldman/flow,claudiopro/flow,MichaelDeBoey/flow,JonathanUsername/flow,jeffmo/flow,popham/flow,ylu1317/flow,AprilArcus/flow,ForbesLindesay/flow,popham/flow,facebook/flow,fletcherw/flow,mroch/flow,TiddoLangerak/flow,facebook/flow,AgentME/flow,popham/flow,jamesgpearce/flow,jamesgpearce/flow,nmote/flow,samwgoldman/flow,jeffmo/flow,JonathanUsername/flow,gabelevi/flow,mroch/flow