Add babel-polyfill

» node
> ripple = require('ripple-lib')
Error: Cannot find module 'babel-core/polyfill'
    at Function.Module._resolveFilename (module.js:336:15)
    at Function.Module._load (module.js:278:25)
    at Module.require (module.js:365:17)
    at require (module.js:384:17)
    at Object.<anonymous> (/Users/alan/Projects/ripple-connect/node_modules/ripple-lib/dist/npm/index.js:23:3)
    at Module._compile (module.js:460:26)
    at Object.Module._extensions..js (module.js:478:10)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Module.require (module.js:365:17)

package.json

@@ -16,6 +16,7 @@
   },
   "dependencies": {
     "ajv": "^1.4.8",
+    "babel-polyfill": "^6.2.0",
     "babel-runtime": "^5.5.4",
     "bignumber.js": "^2.0.3",
     "https-proxy-agent": "^1.0.0",