run Go 1.11 gofmt, use Go 1.10.x and 1.11.x on Travis

.travis.yml

 language: go
 
 go:
-  - "1.9.x"
   - "1.10.x"
+  - "1.11.x"
 
 # first part of the GOARCH workaround
 # setting the GOARCH directly doesn't work, since the value will be overwritten later