Add travis support for osx and optimize CI time.
Add travis support for osx and optimize CI time.


Former-commit-id: 49162d90d8bfb973e9c1e72ee816d6adec27ead4
chunyang-wen/dynet,clab/cnn,clab/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,xunzhang/dynet,clab/dynet,danielhers/dynet,clab/dynet,chunyang-wen/dynet,danielhers/dynet,chunyang-wen/dynet,xunzhang/dynet,danielhers/dynet,shuheik/dynet,xunzhang/dynet,shuheik/dynet,xunzhang/dynet,clab/cnn,danielhers/dynet,chunyang-wen/dynet,clab/cnn,shuheik/dynet,clab/dynet,danielhers/dynet,danielhers/dynet