Fixes Error "Cannot find module"

Webpack cannot find module because it points to non-existing index.js ``` Runtime Error Cannot find module "ng2-gridstack" Stack Error: Cannot find module "ng2-gridstack" at Object.257 (http://localhost:8100/build/main.js:371:7) at __webpack_require__ (http://localhost:8100/build/vendor.js:55:30) at Object.252 (http://localhost:8100/build/main.js:354:70) at __webpack_require__ (http://localhost:8100/build/vendor.js:55:30) at webpackJsonpCallback (http://localhost:8100/build/vendor.js:26:23) at http://localhost:8100/build/main.js:1:1 ```

Author: luckylooke

package.json

@@ -2,7 +2,7 @@
  "name": "ng2-gridstack",
  "version": "0.2.1",
  "description": "A gridstack component for Angular2",
- "main": "index.js",
+ "main": "ng2-gridstack.js",
  "scripts": {
  "prepublish": "tsc"
  },