Use https for open.edx.org

cms/templates/widgets/footer.html

@@ -55,7 +55,7 @@ from openedx.core.djangolib.markup import HTML, Text
       </div>
 
       <div class="footer-about-openedx">
-        <a href="http://open.edx.org" title="${_("Powered by Open edX")}">
+        <a href="https://open.edx.org" title="${_("Powered by Open edX")}">
           <img alt="${_("Powered by Open edX")}" src="https://files.edx.org/openedx-logos/edx-openedx-logo-tag.png">
         </a>
       </div>