Use path-exists

Because fs.exists() and fs.existsSync() is being deprecated

https://github.com/sindresorhus/path-exists

lib/config/concat.js

@@ -1,6 +1,6 @@
 'use strict';
 var path = require('path');
-var fs = require('fs');
+var pathExists = require('path-exists');
 var _ = require('lodash');
 
 exports.name = 'concat';
@@ -27,7 +27,7 @@ exports.createConfig = function (context, block) {
     if (_.isArray(context.inDir)) {
       context.inDir.every(function (d) {
         var joinedPath = path.join(d, f);
-        var joinedPathExists = fs.existsSync(joinedPath);
+        var joinedPathExists = pathExists.sync(joinedPath);
         if (joinedPathExists) {
           files.src.push(joinedPath);
         }

package.json

@@ -26,7 +26,8 @@
   "dependencies": {
     "chalk": "^1.0.0",
     "debug": "^2.1.3",
-    "lodash": "^3.6.0"
+    "lodash": "^3.6.0",
+    "path-exists": "^1.0.0"
   },
   "devDependencies": {
     "grunt": "^0.4.5",