codec: added go 1.11 to travis CI

.travis.yml

@@ -5,6 +5,7 @@ go:
     - 1.8.x
     - 1.9.x
     - 1.10.x
+    - 1.11.x
     - tip
 script:
     - go test -tags "alltests" -run Suite -coverprofile coverage.txt github.com/ugorji/go/codec