Fix GitHub Pages build warning

Currently, the build triggers this warning:

> You are currently using the Jekyll Bootstrap framework which has a known incompatibility with Jekyll v3.1.
> To fix this incompatibility, change `page.theme.name` in `_includes/JB/setup` to `layout.theme.name`.

This commit implements the suggested fix.

Author: Potherca

_includes/JB/setup

@@ -16,7 +16,7 @@
     {% if site.JB.ASSET_PATH %}
       {% assign ASSET_PATH = site.JB.ASSET_PATH %}
     {% else %}
-      {% capture ASSET_PATH %}{{ BASE_PATH }}/assets/themes/{{ page.theme.name }}{% endcapture %}
+      {% capture ASSET_PATH %}{{ BASE_PATH }}/assets/themes/{{ layout.theme.name }}{% endcapture %}
     {% endif %}  
   {% endif %}
-{% endcapture %}{% assign jbcache = nil %}
+{% endcapture %}{% assign jbcache = nil %}