Update changelog

Author: kpeeters

web2/cadabra2/source/index.html

@@ -20,7 +20,7 @@ <h1>News</h1>
   if you want to be a beta tester.
 </p>
 <dl>
-  <dt>19 December 2018</dt>
+  <dt>18 April 2019</dt>
   <dd>Release of 2.2.6
   (<a href="changelog.html#2.2.6">changes</a>). Many features for
   implicit indices, possibility to build as Jupyter kernel)</dd>