Fixed path normalization in Adapter\Dropbox::normalizeObject()

I use Dropbox adapter with directory prefix set. There was an issue with directory contents listing due to paths being prepended with `/` for content entries returned by adapter. All files were omitted in [this line](https://github.com/thephpleague/flysystem/blob/master/src/Cache/AbstractCache.php#L83) because `$object['dirname']` and `$directory` paths were different (`$object['dirname']` contained `/` on the beginning). This small fix resolved this issue.