Update required version of Xcode in README.
Update required version of Xcode in README.

billCTG/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios,billCTG/braintree_ios,billCTG/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios,braintree/braintree_ios,braintree/braintree_ios