Update license to CC BY 4.0

Author: kbroman

License.md

@@ -1,2 +1,4 @@
-My parts of this web site is licensed under
-[CC BY](https://creativecommons.org/licenses/by/3.0/).
+This web site is licensed under
+[CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).
+
+[![CC BY 4.0](https://licensebuttons.net/l/by/4.0/88x31.png)](https://creativecommons.org/licenses/by/4.0/)

ReadMe.md

@@ -11,7 +11,7 @@ View the thing [here](https://kbroman.org/github_tutorial).
 
 ---
 
-My parts of this web site are licensed under
-[CC BY](https://creativecommons.org/licenses/by/3.0/).
+This web site is licensed under
+[CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).
 
-[![CC BY](https://i.creativecommons.org/l/by/3.0/88x31.png)](https://creativecommons.org/licenses/by/3.0/)
+[![CC BY 4.0](https://licensebuttons.net/l/by/4.0/88x31.png)](https://creativecommons.org/licenses/by/4.0/)

_includes/themes/twitter/default.html

@@ -46,7 +46,7 @@
       <footer>
         <p><small>
   <!-- start of Karl's footer; modify this part -->
-          <a href="https://creativecommons.org/licenses/by/3.0/"><img src="https://i.creativecommons.org/l/by/3.0/88x31.png" alt="CC BY"/></a> &nbsp;
+          <a href="https://creativecommons.org/licenses/by/4.0/"><img src="https://licensebuttons.net/l/by/4.0/88x31.png" alt="CC BY"/></a> &nbsp;
           <a href="https://kbroman.org">Karl Broman</a>
   <!-- end of Karl's footer; modify this part -->
         </small></p>