Add unpkg field to package.json (reduxjs#3003)

This allows unpkg to point to the UMD build of redux via https://unpkg.com/redux. Currently this points at lib/redux.js which will not run in browsers.

package.json

@@ -12,6 +12,7 @@
     "Andrew Clark <[email protected]> (https://github.com/acdlite)"
   ],
   "main": "lib/redux.js",
+  "unpkg": "dist/redux.js",
   "module": "es/redux.js",
   "typings": "./index.d.ts",
   "files": [