Add missing argument to Errorf()

This resulted in the error message (when the test failed) displaying all the
values offset by one and the last one missing. Identified by `go vet`.
Fix format string type error

The expected value (expectedHeaderVals) is a string not a bool. Identified
by `go vet`.
Run `go vet` in Travis CI

To prevent further formatting errors like the last two commits.
Install `go vet` in TravisCI

This isn't available in the version of Go on Travis. It was split from the
standard distribution at some point. So install it before we require it. I'm
putting it in the script rather than `.travis.yml` for transparency.
Merge pull request #100 from alphagov/go_vet_fixes

Go vet fixes