updated the appveyor yml file to only build master for the main CI bulid
updated the appveyor yml file to only build master for the main CI bulid

RestSharp-resurrected/RestSharp,amccarter/RestSharp,dyh333/RestSharp,benfo/RestSharp,jiangzm/RestSharp,mattleibow/RestSharp,rucila/RestSharp,felipegtx/RestSharp,kouweizhong/RestSharp,KraigM/RestSharp,cnascimento/RestSharp,rivy/RestSharp,periface/RestSharp,restsharp/RestSharp,mattwalden/RestSharp,eamonwoortman/RestSharp.Unity,huoxudong125/RestSharp,lydonchandra/RestSharp,mwereda/RestSharp,uQr/RestSharp,PKRoma/RestSharp