just rely on path.join to insert the correct sep

NodeJS 0.6 doesn't support path.sep as that was introduced in Node 0.7,
so let's not use that.  Instead, just rely on path.join to do the right
thing as that's what it's there for.

tasks/coffee.js

@@ -44,7 +44,7 @@ module.exports = function(grunt) {
     extension = typeof extension === "undefined" ? '.js' : extension;
 
     if( destPath && options.preserve_dirs ){
-      var dirname = path.normalize(path.dirname(src)) + path.sep;
+      var dirname = path.normalize(path.dirname(src));
       if ( options.base_path ) {
         //on Windows, path.sep must be escaped in a regex
         var safeRegex = path.normalize(options.base_path).replace(/\\/g, "\\\\");