Broken/dead links found in README

[tracedynamics]

Hi! I found some broken links in the README:

1. http://exploringjs.com/es2016-es2017.html - 404 Not Found
2. http://exploringjs.com/es6.html - 404 Not Found
3. http://speakingjs.com/ - DNS resolution failed (domain no longer resolves)
4. https://cdn.rawgit.com/voodootikigod/logo.js/master/js.svg - 404 (badge image, minor)

The exploringjs.com domain appears to have moved their content. Would you be open to a PR that updates these links with working alternatives? Happy to submit one.

[mischah]

Yep. A PR is highly appreciated. Thanks