switch dependency to newer phantom

[gasolin]

I saw a phantomjs-prebuilt (v2) package is available on npm https://www.npmjs.com/package/phantomjs-prebuilt

So I modify hubot-skype-web/src/skypewebb.coffee and use phantom = require('phantomjs-prebuilt'); instead.

The following error occurred:

TypeError: phantom.create is not a function
  at SkypeWebAdapter.login (/bot/node_modules/hubot-skype-web/src/skypeweb.coffee:118:13)

[gasolin]

The issue is occurred because the dependent phantom package version is not updated https://github.com/amir20/phantomjs-node

And phantom-node is changed to promise based syntax