Update contributors

Change-Id: I6bff2f54137664f15a9f71a8a9cbd54d3afb9c54
Signed-off-by: Daniel Strueber <strueber@mathematik.uni-marburg.de>
diff --git a/contributors.php b/contributors.php
index 05fa2bf..cdd9fa2 100644
--- a/contributors.php
+++ b/contributors.php
@@ -36,9 +36,10 @@
 <ul>
 <li><a href="https://www.uni-marburg.de/fb12/arbeitsgruppen/swt/kristopher-born">Kristopher Born</a></li>
 <li><a href="http://www.frankhermann.eu/">Frank Hermann</a></li>
-<li>Timo Kehrer</a></li>
+<li><a href="https://www.informatik.hu-berlin.de/de/org/mitarbeiter/1689904">Timo Kehrer</a></li>
 <li><a href="http://www.ckrause.org">Christian Krause</a></li>
-<li><a href="https://rgse.uni-koblenz.de/web/pages/people/strueber/index_en.shtml">Daniel Str&uuml;ber</a> (project lead)</li>
+<li><a href="https://rubyfi.de/">Stefan Schulz</a></li>
+<li><a href="https://www.danielstrueber.de">Daniel Str&uuml;ber</a> (project lead)</li>
 <li><a href="https://www.uni-ulm.de/in/pm/mitarbeiter/tichy.html">Matthias Tichy</a></li>
 </ul>
 </p>