Added `generator-mocha` to `peerDependencies`

Added generator-mocha dependency into npm peerDependencies as discussed in yeoman/yo#19. This is in preparation for removing default generators from `yo`.

If you want to test it out locally, you can install it directly from my branch using this command:

```bash
$ npm install https://nodeload.github.com/danielmcormond/generator-webapp/tar.gz/peer-dependencies
```