Add Go 1.8 to the travis config.

Go 1.8 and 1.9 use different text for the connection closed error.
Use their common prefix (i.e., "use of closed") in the tests for
them to pass on all Go versions.

Go 1.8: "use of closed network connection"
Go 1.9: "use of closed file or network connection"

Suggested-by: Damien Neil <dneil@google.com>

.travis.yml

@@ -4,6 +4,7 @@ go:
   - 1.5
   - 1.6
   - 1.7
+  - 1.8
   - tip
 
 matrix:

cmux_test.go

@@ -42,7 +42,7 @@ const (
 )
 
 func safeServe(errCh chan<- error, muxl CMux) {
-	if err := muxl.Serve(); !strings.Contains(err.Error(), "use of closed network connection") {
+	if err := muxl.Serve(); !strings.Contains(err.Error(), "use of closed") {
 		errCh <- err
 	}
 }