http://github.com -> https://github.com

Change http://github.com to https://github.com to avoid plaintext HTTP connections.

README.md

@@ -1,6 +1,6 @@
 ![Atom](https://cloud.githubusercontent.com/assets/72919/2874231/3af1db48-d3dd-11e3-98dc-6066f8bc766f.png)
 
-Atom is a hackable text editor for the 21st century, built on [atom-shell](http://github.com/atom/atom-shell), and based on everything we love about our favorite editors. We designed it to be deeply customizable, but still approachable using the default configuration.
+Atom is a hackable text editor for the 21st century, built on [atom-shell](https://github.com/atom/atom-shell), and based on everything we love about our favorite editors. We designed it to be deeply customizable, but still approachable using the default configuration.
 
 Visit [atom.io](https://atom.io) to learn more or visit the [Atom forum](https://discuss.atom.io).