Use custom .npmrc for installing grunt and grunt-cli, too

Npmjs recently [disallowed self-signed certs](http://blog.npmjs.org/post/78085451721/npms-self-signed-certificate-is-no-more) which means that to deploy to heroku, we need to explicitly include:

```
ca=''
```

in our `.npmrc` files. The `npm install` step works fine, but `npm install grunt-cli` and `npm install grunt` break because they don't use custom `.npmrc`s -- this pull request fixes that.

Thanks!