Add cross-compilation to Travis

As noted in https://github.com/docker/swarm/issues/209#issuecomment-68330145, this excludes Windows for now (`api/server.go:29: undefined: syscall.Umask`), and also excludes Go 1.4 for now (https://github.com/moovweb/gvm/pull/116), but does include everything necessary to reenable them later without much fuss (once the issues with each are respectively resolved).