Add babel-runtime for Symbol

package.json

@@ -5,7 +5,7 @@
   "main": "lib/index.js",
   "scripts": {
     "clean": "rimraf lib",
-    "build": "babel src --out-dir lib",
+    "build": "babel src --optional runtime --out-dir lib",
     "lint": "eslint src test examples",
     "test": "NODE_ENV=test mocha --compilers js:babel/register --recursive",
     "test:watch": "NODE_ENV=test mocha --compilers js:babel/register --recursive --watch",
@@ -47,6 +47,7 @@
     "react": ">=0.13.3 || ^0.14.0-beta3"
   },
   "dependencies": {
+    "babel-runtime": "^5.8.20",
     "react-mixin": "^1.7.0"
   }
 }