Update travis.yml to use Xcode 8.3 image
Update travis.yml to use Xcode 8.3 image

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