Update README.md Carthage instructions (#139)
Update README.md Carthage instructions (#139)

Summary: Update README.md Carthage instructions in response to OASIS-1358

Test Plan:
CarthageTest app was built and tested according to new instructions
and PASSED as mentioned in OASIS-1358 JIRA comment.

Reviewers: alda

JIRA Issues: OASIS-1358

Differential Revision: https://phabricator.optimizely.com/D16175
optimizely/objective-c-sdk,optimizely/objective-c-sdk,optimizely/objective-c-sdk,optimizely/objective-c-sdk