Add deployment target to podspec.
Add deployment target to podspec.

Summary: Without specifying the deployment target, it was default to iOS 4.3. This would cause a build error because we are `unable to synthesize weak properties` which didn't come around until iOS 5. Setting default target to iOS 8.

Reviewed By: emilsjolander

Differential Revision: D4481772

fbshipit-source-id: d3be88d0f73533c6242fc1d9045c70ad4628fd21

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