Fix travis build
Fix travis build

Summary: Building on osx machines makes things a lot easier.

Differential Revision: D3693987

fbshipit-source-id: f6e2599f99a3fd4ac6fb2a6efd5563074da707d0

facebook/css-layout,yihuang/css-layout,rmarinho/yoga,facebook/yoga,rmarinho/yoga,yihuang/css-layout,yihuang/css-layout,facebook/css-layout,yihuang/css-layout,rmarinho/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/css-layout,rmarinho/yoga,rmarinho/yoga,facebook/yoga,rmarinho/yoga,facebook/yoga,facebook/css-layout,facebook/css-layout,yihuang/css-layout,facebook/yoga,yihuang/css-layout,facebook/yoga,facebook/css-layout,yihuang/css-layout,rmarinho/yoga,facebook/yoga,rmarinho/yoga,facebook/yoga,yihuang/css-layout,facebook/css-layout,rmarinho/yoga,facebook/yoga,facebook/yoga,yihuang/css-layout,yihuang/css-layout,rmarinho/yoga