Travis: Check for gofmt issues (#12)

The default test script is `go test -v ./...` according to Travis docs at https://docs.travis-ci.com/user/languages/go/#Default-Test-Script. Add an additional gofmt check to help catch formatting issues in PRs. This is motivated by a previous occurrence at #9 (comment). Also swap the order of Go versions 1.x and 1.6. 1.x is higher priority, so it makes more sense to run it first and report issues right away. 1.6 is lower priority, so it's better to prioritize it lower. Use 2 space indent in the .yml file, which is consistent with the formatting in Travis documentation and examples.

Author: dmitshur

.travis.yml

@@ -1,5 +1,8 @@
 sudo: false
 language: go
 go:
-- 1.6
-- 1.x
+ - 1.x
+ - 1.6
+script:
+ - diff -u <(echo -n) <(gofmt -d .)
+ - go test -v ./...