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

[1]: https://creativecommons.org/version4
Merge pull request #96 from r-gaia-cs/cc-by-4

Upgrade to CC BY 4.0