[deps] Update all deps and drop support for node < 6

[noratarano]

Summary

Update all dependencies. We also drop support for node versions < 6 since babel dropped support for them in babel@7.

Note: This is step 2 in a list of TODOs that I have for refactoring this library to not use singletons. While these updates are not necessary, they keep the repo modernized.

• [x] 1. ~Update react-with-styles deps~: https://github.com/airbnb/react-with-styles/pull/219
• [x] 2. ~Update react-with-styles-interface-aphrodite deps~: https://github.com/airbnb/react-with-styles-interface-aphrodite/pull/65
• [ ] 3. Update react-with-styles to use styles interface and theme in context
• [ ] 4. Publish react-with-styles v4.0.0
• [ ] 5. Update the react-with-styles-interface-aphrodite peer dep version for react-with-styles
• [ ] 6. Publish react-with-styles-interface-aphrodite v6.0.0

Testing

• [x] Ran npm run build and npm run test using node@8.9.1 and npm@6.9.0 with success
• [x] CI tests all the other versions of node and npm that we support

@brucewpaul @goatslacker @ljharb @lencioni