Drop support for Snow Leopard

* Snow Leopard was released in August, 2009 (about five years ago).
* Yosemite will be released in a couple of months.
* We support the last three versions of Ubuntu, so this is a little more
  consistent.
* Use the same format as the Linux "Requirements" section for consistency.
Link README to programs that Laptop installs

So that users can learn more about them.