chore(deps): used peerDependencies to install generator-karma

Moved `generator-karma` dependency into npm `peerDependencies` as discussed in https://github.com/yeoman/yo/issues/19.

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-angular/tar.gz/peer-dependencies
```