Enable cocoapods caching (#491)

Summary:
Enable caching for cocoapods.

This was my attempt to fix the test which priteshrnandgaonkar did with https://github.com/facebook/flipper/issues/490, but might still be useful.
Pull Request resolved: https://github.com/facebook/flipper/pull/491

Reviewed By: priteshrnandgaonkar

Differential Revision: D16280689

Pulled By: passy

fbshipit-source-id: 5da72a7b75d9f49afa8f6c889074bf31e9d58dcd

.travis.yml

@@ -41,6 +41,8 @@ matrix:
 
     - language: objective-c
 
+      cache: cocoapods
+
       before_install:
         - rm -rf iOS/Sample/Pods
         - rm -f iOS/Sample/Podfile.lock