Use singular repository to avoid warning

Hi! It appears having the plural `repositories` in your package.json will give a warning:

```
npm WARN package.json eventemitter2@0.4.11 'repositories' (plural) Not supported.
npm WARN package.json Please pick one as the 'repository' field
```

This updates to use the singular `repository`. Reference isaacs/npm#2624 and gruntjs/grunt-lib-contrib#8

Thanks!