`Berksfile#vendor` [requires](https://github.com/RiotGames/berkshelf/blob/7e840736aec441721fa17badc75d6d58ddce414c/lib/berkshelf/berksfile.rb#L610-L613) that the directory won't exist, so I remove it with `FileUtils#rm_rf`.

Saying that I just realize that the tempdir is not removed after the test run (neither before or after this PR). :/