Repair cask vlc-nightly: App source is not there (#2398)

Error: It seems the App source is not there: '/usr/local/Caskroom/vlc-nightly/latest/vlc-3.0.0-git/VLC.app'

- Added correct container type `dmg`
- Added target to allow installation alongside current VLC.app, sharing prefs.
- Added two caveats(!)

Remove caveats and target

Casks/vlc-nightly.rb

@@ -9,5 +9,5 @@ cask 'vlc-nightly' do
 
   depends_on macos: '>= :lion'
 
-  app 'vlc-3.0.0-git/VLC.app'
+  app 'VLC.app'
 end