Using the example from the docs, I can successfully write to my repo.

// example write
repo.write('master', 'path/to/file', 'YOUR_NEW_CONTENTS', 'YOUR_COMMIT_MESSAGE', function(err) {});

But only the first save seems to work. The others return a 422 status with the following error. I did notice this prose ticket...wondering what the workaround there was?

{
  "message": "Update is not a fast forward",
  "documentation_url": "http://developer.github.com/v3"
}