fix parsing of GitHub repo paths

GitHub usernames cannot start with an underscore, but they can be a
single character long.

autoload/vundle/config.vim

@@ -41,7 +41,7 @@ endf
 func! s:parse_name(arg)
   let arg = a:arg
   if    arg =~? '^\s*\(gh\|github\):\S\+'
-  \  || arg =~? '^\w[a-z0-9-]\+/[^/]\+$'
+  \  || arg =~? '^[a-z0-9][a-z0-9-]*/[^/]\+$'
     let uri = 'https://github.com/'.split(arg, ':')[-1]
     let name = substitute(split(uri,'\/')[-1], '\.git\s*$','','i')
   elseif arg =~? '^\s*\(git@\|git://\)\S\+'