Fix loaded task checking

At present, *jit-grunt* fails to load the plugin [grunt-contrib-concat](https://github.com/gruntjs/grunt-contrib-concat), because the array of loaded task names contains [`concat` in its prototype](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/concat) and *jit-grunt* regards `concat` task as already loaded.
So I introduced [`indexOf`](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf) to check if an array has a value without fail.