Update changelog for 3.0.0 release
Update changelog for 3.0.0 release

https://trello.com/c/4iWPptdD

braintree/braintree_ios,EverettQuebral/braintree_ios,bytemarkinc/braintree_ios,Feverup/braintree_ios,TeachersPayTeachers/braintree_ios,apascual/braintree_ios,billCTG/braintree_ios,Feverup/braintree_ios,EverettQuebral/braintree_ios,billCTG/braintree_ios,bytemarkinc/braintree_ios,TeachersPayTeachers/braintree_ios,TeachersPayTeachers/braintree_ios,mickeyreiss/braintree_ios,billCTG/braintree_ios,bytemarkinc/braintree_ios,Feverup/braintree_ios,apascual/braintree_ios,TeachersPayTeachers/braintree_ios,bytemarkinc/braintree_ios,EverettQuebral/braintree_ios,bytemarkinc/braintree_ios,apascual/braintree_ios,braintree/braintree_ios,Feverup/braintree_ios,EverettQuebral/braintree_ios,EverettQuebral/braintree_ios,billCTG/braintree_ios,TeachersPayTeachers/braintree_ios,mickeyreiss/braintree_ios,apascual/braintree_ios,braintree/braintree_ios,mickeyreiss/braintree_ios,mickeyreiss/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios