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.

pkgs/misc/vim-plugins/default.nix

@@ -115,7 +115,7 @@ in rec
       sha256 = "1yxrxskxnr2da5awm59ra4s9wg67rimcbazvln9bayg9saxs540d";
      };
 
-    name = "youcompleteme-git-a2cae90";
+    name = "youcompleteme-20140929";  # commit date
     buildInputs = [ python cmake clang.clang ];
 
     configurePhase = ":";