Add VLC.app 2.2.0-20140520 (nightly build)

The VLC nightlies site does offer a `last` URL pointing to the most recent nightly build, but neither content type nor extension indicate that this is a zip archive, so it does not seem usable with cask (unless one can specify the content type in the formula).

I used the date in the version string (which is also in the URL) even though VLC.app is built as version `2.2.0-git`.