[fixed] Add file extention to entry point (#294)

http://stackoverflow.com/questions/41077055/typescript-2-1-feature-of-implicit-import-not-working

Author: kenju

Diff for: package.json

@@ -2,7 +2,7 @@
   "name": "react-modal",
   "version": "1.6.4",
   "description": "Accessible modal dialog component for React.JS",
-  "main": "./lib/index",
+  "main": "./lib/index.js",
   "repository": {
     "type": "git",
     "url": "https://github.com/reactjs/react-modal.git"