Update for Vundle's new interface

See https://github.com/gmarik/Vundle.vim/blob/master/doc/vundle.txt#L345-L369
Ugh, you're right. I avoided changing the name of the local directory for backwards compatibility, but forgot that Vundle will just name the folder the same thing as whatever's defined in the bundles file.

Should I just change the folder from `$HOME/.vim/bundle/vundle` to `$HOME/.vim/bundle/Vundle.vim` (and possibly `rm -rf` the old directory)? This has the added benefit of removing the specific revision check since everyone who runs rcup after this is merged will be pulling the latest version of Vundle.