Validate icon names

[insin]

• [ ] Update and pin octicons version
• [x] Determine icon names from node_modules/octicons/ at build time and inject with Webpack's DefinePlugin (meh, let's just hard-code it like the demo)
• [x] Move propTypes stuff (+the icon names list) into a if (process.env.NODE_ENV !== 'production') block (nwb does this for you now)
• [x] Enable GreenKeeper on this repo (no point doing this now as versions beyond 4 no longer provide an icon font)