Fixing missing reference to path in lib/core/mount.js

`lib\core\mount.js` has a reference to `path` on [line 218](https://github.com/JedWatson/keystone/blob/master/lib/core/mount.js#L218), however it's missing the `require('path')`. This causes an error only when attempting to mount Keystone as a sub-app,  without setting the `views` option.