bump min version of ocaml
bump min version of ocaml

Summary:
Came across when trying to compile infer on fresh VM.
Infer depends on reason now. Reason requires ocaml 4.02.3

Reviewed By: jberdine

Differential Revision: D3371378

fbshipit-source-id: 598c602

jsachs/infer,facebook/infer,facebook/infer,ropas/infer,kageiit/infer,xennygrimmato/infer,ropas/infer,jsachs/infer,ryoon/infer,blzq/infer,ropas/infer,jvillard/infer,ropas/infer,jvillard/infer,kageiit/infer,jsachs/infer,xennygrimmato/infer,blzq/infer,jsachs/infer,jvillard/infer,ryoon/infer,xennygrimmato/infer,ropas/infer,facebook/infer,jvillard/infer,facebook/infer,xennygrimmato/infer,jvillard/infer,jsachs/infer,blzq/infer,blzq/infer,kageiit/infer,facebook/infer,jsachs/infer,facebook/infer,jvillard/infer,ropas/infer,xennygrimmato/infer,blzq/infer,facebook/infer,xennygrimmato/infer,ryoon/infer,ryoon/infer,kageiit/infer,kageiit/infer,blzq/infer,jvillard/infer,xennygrimmato/infer,kageiit/infer,ryoon/infer,ryoon/infer,kageiit/infer