Update CHANGELOG for 4.15.1.
Update CHANGELOG for 4.15.1.

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