.travis.yml: Only run 'gofmt' on 1.11.x

The indent heuristic changed with Go 1.11 [1], so there's no single
form that satisfies gofmt for both Go 1.10 and Go 1.11.  With this
commit, we now only run gofmt on 1.11.

Docs for the 'include' syntax are in [2].

Also replace the double-diff with a 'gofmt -w ...' to write the
changes and a 'git diff ...' call to display the changes and exit
non-zero if there were any.

[1]: https://golang.org/doc/go1.11#gofmt
[2]: https://docs.travis-ci.com/user/customizing-the-build/#explicitly-including-jobs

.travis.yml

@@ -1,17 +1,18 @@
 sudo: false
 language: go
-go:
-  - 1.10.x
-  - 1.11.x
-  - master
 matrix:
   allow_failures:
     - go: master
   fast_finish: true
+  include:
+    - go: 1.10.x
+    - go: 1.11.x
+      env: GOFMT=1
+    - go: master
 install:
   - # Do nothing. This is needed to prevent default install action "go get -t -v ./..." from happening here (we want it to happen inside script step).
 script:
   - go get -t -v ./...
-  - diff -u <(echo -n) <(gofmt -d -s .)
+  - if test -n "${GOFMT}"; then gofmt -w -s . && git diff --exit-code; fi
   - go tool vet .
   - go test -v -race ./...