remove Go 1.7 add Go 1.11

.travis.yml

@@ -1,10 +1,10 @@
 language: go
 
 go:
-  - 1.7.x
   - 1.8.x
   - 1.9.x
   - 1.10.x
+  - 1.11.x
   - master
 
 go_import_path: gopkg.in/jcmturner/gokrb5.v6