Bump up the supported Go version in .travis.yml

Author: mingrammer

.travis.yml

@@ -1,11 +1,9 @@
 language: go
 
 go:
-  - "1.6"
-  - "1.7"
-  - "1.8"
-  - "1.9"
-  - "1.10"
+  - "1.11.x"
+  - "1.12.x"
+  - "1.13.x"
   - master
 
 script: go test -race -coverprofile=coverage.txt -covermode=atomic