Rename OutputWrite.WriteTo method to Write

Avoids:
 go vet: error should have signature WriteTo(io.Writer) (int64, error)
Add go vet to travis script

Skipping (forked) xml package due to go vet errors such as:

error should have signature MarshalXML(*xml.Encoder, xml.StartElement) error
Install go vet for travis
Merge pull request #77 from dougm/vet

Add go vet to travis script