generator.remote();  added --force flag to tell npm to fetch upstream even when cache hit

I have a simple generator that is just pulling a pre-set boilerplate with yo.  The problem is that when the boilerplate repo is updated, yo.remote() does not perform a git fetch.

Here is the remote call:
https://github.com/mixdown/generator-mixdown/blob/master/app/index.js#L25

To resolve, I have to tell users to do the following:

```
rm -rf  /Users/your-user-name/.cache/yeoman/mixdown
```

*From npm docs*
The --force argument will force npm to fetch remote resources even if a local copy exists on disk.