Summary:
This PR updates the installation instruction for Cocoapods 1.7+ as the previous installation instruction was broken for the latest Cocoapods version
Solves https://github.com/facebook/flipper/issues/478 and https://github.com/facebook/flipper/issues/495
## Changelog
- Updated the Documentation
- Updated the sample app
Pull Request resolved: https://github.com/facebook/flipper/pull/522
Test Plan:
- Travis CI
- Also tested locally by building sample app.
Reviewed By: jknoxville
Differential Revision: D16902023
Pulled By: priteshrnandgaonkar
fbshipit-source-id: 19cc01c842f93ab3565c302f8258ca88dbdecfb2