Change license to CC BY 4.0

Based on @wking's work.

## Steps to reproduce

~~~
$ cd lesson-template
$ make preview
$ cd site
~~~

Open `../lesson-template/LICENSE.html` to copy the "Instructional Material" section and replace it on `license.html`.
On Sat, Dec 20, 2014 at 09:33:21AM -0800, Greg Wilson wrote:
> >  </ul>
> > -<p>Under the following conditions:</p>
> > +<p>for any purpose, even commercially.</p>
> > +<p>The licensor cannot revoke these freedoms as long as you follow the license terms.</p>
> > +<p>Under the following terms:</p>
> 
> This line appears out of place - it's not a full sentence, doesn't
> follow from what's immediately above it, and doesn't lead into what
> follows.  Please check the original.

It follows from the previous unindented content (see [1]).

[1]: https://github.com/swcarpentry/workshop-template/pull/96/files#r22142807