Merge pull request #1572 from michaelbazos/linkfixes

Fixed link for ebook "Exploring ES6"
thanks for the updated link

free-programming-books.md

@@ -1179,7 +1179,7 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
 * [Dev Docs](http://devdocs.io/)
 * [Developing Backbone.js Applications](http://addyosmani.github.io/backbone-fundamentals/) - Addy Osmani
 * [Eloquent JavaScript 2nd edition](http://eloquentjavascript.net/) - Marijn Haverbeke
-* [Exploring ES6](https://leanpub.com/exploring-es6/read) - Dr. Axel Rauschmayer
+* [Exploring ES6](http://exploringjs.com/es6/) - Dr. Axel Rauschmayer
 * [Google JavaScript Style Guide](http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml)
 * [Human Javascript](http://read.humanjavascript.com/)
 * [JavaScript Allongé](https://leanpub.com/javascript-allonge/read) - Reginald Braithwaite