Fix npm "dependencies" warning

npm is issuing the following warning when using JSV:

```
npm WARN package.json JSV@4.0.2 'repositories' (plural) Not supported.
npm WARN package.json Please pick one as the 'repository' field
```

This commit changes the `repositories` key in `package.json` to `repository` as
per the new npm package schema:

https://npmjs.org/doc/files/package.json.html

#78 is a very similar pull request, except it doesn't remove the array.