vim-plugins: coffee-script: start version number with digit

Or else nix will see it as part of package name.
vim-plugins: use commit dates instead of commit ids as version numbers

Git commit ids shouldn't be used as version numbers in nixpkgs:
* they don't increase monotonically
* they don't always start with a digit, causing nix to not parse it as
  a version number, but instead sees it as  part of the package name,
  causing the package to (seemingly) conflict with itself upon upgrade.