Update go versions for travis-ci

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 go:
-  - "1.9.4"
-  - "1.10.1"
-  - "1.11.2"
+  - "1.10.8"
+  - "1.11.8"
+  - "1.12.3"
   - tip
 
 before_install: