Update dependency underscore.js to v1.13.7

Author: renovate[bot]

_includes/head.html

@@ -15,7 +15,7 @@
   <link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/font-mfizz/2.4.1/font-mfizz.min.css">
 
   <script defer src="//code.jquery.com/jquery-2.2.3.min.js"></script>
-  <script defer type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore-min.js"></script>
+  <script defer type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/underscore.js/1.13.7/underscore-min.js"></script>
   <script defer type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.30.1/moment.min.js"></script>
   <script defer src="{{ site.baseurl }}/assets/js/bootstrap.min.js"></script>
   <script type="text/javascript" src="https://steadyhq.com/widget_loader/7bc62501-0396-4ad8-974f-797844736877"></script>