Change gitcdn link (alshedivat#234)

gitcdn.link was quite slow.  Use alternate link at gitcdn.xyz.

Author: jkmacc-LANL

_includes/head.html

@@ -25,7 +25,7 @@
 <link rel="stylesheet" type="text/css" href="https://fonts.googleapis.com/css?family=Roboto:300,400,500,700|Roboto+Slab:100,300,400,500,700|Material+Icons">
 
 <!-- Code Syntax Highlighting -->
-<link rel="stylesheet" href="https://gitcdn.link/repo/jwarby/jekyll-pygments-themes/master/{{ site.highlight_theme }}.css" />
+<link rel="stylesheet" href="https://gitcdn.xyz/repo/jwarby/jekyll-pygments-themes/master/{{ site.highlight_theme }}.css" />
 
 <!-- Styles -->
 {% if site.icon != empty %}