[doc] update install instructions
[doc] update install instructions

Summary: Ubuntu 12 is ancient, so delete the instructions for that.

Reviewed By: mbouaziz

Differential Revision: D6089203

fbshipit-source-id: aefd13b

facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer