add jsnext:main field

Author: yyx990803

package.json

@@ -3,6 +3,7 @@
   "version": "0.7.4",
   "description": "A router for Vue.js",
   "main": "lib/index.js",
+  "jsnext:main": "src/index.js",
   "scripts": {
     "dev": "npm run serve & npm run serve-test & webpack --watch --config build/webpack.dev.config.js",
     "lint": "eslint src build test/e2e test/unit/specs",