Provide repository to package.json for WebJars compatibility

[trathschlag]

I am trying to use tiny-invariant with webjars.org and get the following error:

The metadata was missing a required field: /repository/url
This could be easily fixed by adding the information to the package.json:

"repository": {
  "type" : "git",
  "url" : "..."
}

Thanks in advance!

[Alex Reardon]

PR welcome!