Update Travis config for Xcode 9.3 beta.
Update Travis config for Xcode 9.3 beta.

opentable/AFNetworking,E-B-Smith/AFNetworking,nonoBruce/AFNetworking,Kjuly/AFNetworking,E-B-Smith/AFNetworking,FelixYin66/AFNetworking,nonoBruce/AFNetworking,opentable/AFNetworking,AFNetworking/AFNetworking,Kjuly/AFNetworking,sunkehappy/AFNetworking,AFNetworking/AFNetworking,nonoBruce/AFNetworking,Kjuly/AFNetworking,AFNetworking/AFNetworking,opentable/AFNetworking,E-B-Smith/AFNetworking