Use IPv4 for the listener to avoid v6 failures on Travis.

cmux_test.go

@@ -81,7 +81,7 @@ func (l *chanListener) Accept() (net.Conn, error) {
 }
 
 func testListener(t *testing.T) (net.Listener, func()) {
-	l, err := net.Listen("tcp", ":0")
+	l, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		t.Fatal(err)
 	}