[readme] Remove outdated brew cask instruction
[readme] Remove outdated brew cask instruction

Summary: Manually installing brew-cask is no longer necessary.

Reviewed By: jeremydubreil

Differential Revision: D5401661

fbshipit-source-id: ea650e8

blzq/infer,ropas/infer,jvillard/infer,blzq/infer,jvillard/infer,blzq/infer,ropas/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,ropas/infer,blzq/infer,jvillard/infer,jvillard/infer,facebook/infer,ropas/infer,jvillard/infer,ropas/infer,facebook/infer,blzq/infer,ropas/infer,jvillard/infer,blzq/infer,facebook/infer,facebook/infer