Fix Travis gofmt check.

Currently, the Travis test passes, but the gofmt diff step is not testing what was intended:

```
$ diff <(gofmt -d .) <("")
/home/travis/build.sh: line 250: : command not found

The command "diff <(gofmt -d .) <("")" exited with 0.
```

This PR fixes it. I've also swapped the the two diff parameters and added -u flag to make the diff (if something isn't gofmt-ed) more readable.