Update CHANGELOG for 4.18.0
Update CHANGELOG for 4.18.0

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