LICENSE.md: Bump the CC BY 3.0 to 4.0

Titles from the v4 release notes [1]:

* A more global license
* Rights outside the scope of copyright
* Common-sense attribution
* Enabling more anonymity, when desired
* 30-day window to correct license violations
* Increased readability
* Clarity about adaptations

On Wed, Oct 22, 2014 at 09:25:13AM -0400, Greg Wilson wrote:
> On 2014-10-21 1:01 PM, W. Trevor King wrote:
> > On Tue, Oct 21, 2014 at 12:26:27PM -0400, Trevor Bekolay wrote:
> >> https://github.com/tbekolay/swc-template.
> >
> > Can greenfield lessons be CC BY 4.0 (more interantionally robust
> > than the 3.0 collection [1])?
>
> Sure - file a PR against Trevor B's repo?

[1]: https://creativecommons.org/version4