Jag var tvungen att ta bort katalogen från de gamla referenserna:
sudo install_name_tool -change libmysqlclient.18.dylib
/usr/local/mysql/lib/libmysqlclient.18.dylib
/Library/Ruby/Gems/1.8/gems/mysql-2.7/lib/mysql.bundle
sudo install_name_tool -change libmysqlclient.18.dylib
/usr/local/mysql/lib/libmysqlclient.18.dylib
/Library/Ruby/Gems/1.8/gems/mysql-2.7/mysql.bundle