Add hint on FF addon. Fixes #201.

I never update FF so lets inform the user running `grunt ff:watch`
Creating multiple variables for one message is unnecessary. Also the code/error really should be on it's own line:

```js
return grunt.warn('Auto-loading ' + xpi + ' failed:\n\n' +
  code + ': ' + error + '\n\n' +
  'Ensure you have the AutoInstaller extension installed in Firefox:\n' +
  'https://addons.mozilla.org/en-US/firefox/addon/autoinstaller/\n\n'
);
```