Add babel-core dependency and regenerate new shrinkwrap

There were issues with the shrinkwrap process after adding babel-core.
Also, babel-core is required, otherwise this failure was happening on a fresh
install of ripple-lib:

» 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)