tests: run mvdan/shfmt on test runs

While I do not agree with _every_ style change, this will force Dokku to have consistent formatting across all shell scripts, which is arguably a Good Thing™.

The command used to reprocess everything is:

```shell
shfmt -l -bn -ci -i 2 -w .
```

plugins/common/vendor/github.com/codegangsta/inject/update_readme.sh

@@ -1,3 +1,3 @@
 #!/bin/bash
 go get github.com/robertkrimen/godocdown/godocdown
-godocdown > README.md
+godocdown >README.md