Ensure no parameter is passed to 'done' function

Since nodejs 9.0.0, fs.writeFile seems to throw an undefined parameter as error instead of no parameter in the call back. It leads to an error 'done is not a function'

tasks/webfont.js

@@ -318,7 +318,7 @@ module.exports = function(grunt) {
 
 			// Save
 			var woff2FontPath = wf.getFontPath(o, 'woff2');
-			fs.writeFile(woff2FontPath, woffFont, done);
+			fs.writeFile(woff2FontPath, woffFont, function() {done();});
 		}
 
 		/**