Add specific Go versions to test
Add specific Go versions to test

There's an additional field on json.UnmarshalTypeError between 1.4 and
tip (presumably added in 1.5 but I'm running with 1.6beta1 at the
moment), so it makes sense to run tests against 1.4 and 1.5. I don't
really plan on supporting anything older.

nilium/dagr