Bump Go versions and use '.x' to always get latest patch versions

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 go:
-- 1.9.7
-- 1.10.4
-- 1.11
+- 1.9.x
+- 1.10.x
+- 1.11.x
 
 env:
   global: