update copyright to 2020

Author: mprat

_includes/footer.html

@@ -35,7 +35,7 @@ <h2 class="footer-heading">{{ site.title | escape }}</h2>
       </div>
 
       <div class="footer-col">
-        <i>Copyright 2017-2019, Michele Pratusevich, Learnable LLC.</i>
+        <i>Copyright 2017-2020, Michele Pratusevich, Learnable LLC.</i>
         <i>Hosted by <a href="http://pages.github.com" target="_blank">Github Pages.</a></i>
       </div>