youcompleteme (vim plugin): update to latest

patchShebangs fixes build failure in chroot (in this new version).

The --system-libclang flag is now needed to prevent YouCompleteMe from
trying to download and build clang (which fails).
youcompleteme (vim plugin): use commit date as version number

Using a git commit id as version number doesn't work well, so use the
commit date instead.

Also, there is no other version of YouCompleteMe than the git version,
so drop 'git' from the package name.