Update site.gems references to site.plugins (#39)

* Update `site.gems` references to `site.plugins`

Fixes checks to determine if jekyll-seo-tag and jekyll-feed are active.

Related to #24

* Add `site.gems` checks for backwards compatibility

Check both `site.gems` and `site.plugins` when determining if plugins are active.
Allows compatibility with Jekyll versions prior to 3.5.0.

Related to #24

Author: tony-ho

_includes/contact-list.html

@@ -17,7 +17,7 @@
   {% endif %}
 
   <li>
-    {% if site.plugins contains 'jekyll-feed' %}
+    {% if site.plugins contains 'jekyll-feed' or site.gems contains 'jekyll-feed' %}
       <a href="{{ site.feed.path | default: 'feed.xml' | relative_url }}" title="Atom Feed">
         <span class="icon icon--rss">{% include icon-rss.svg %}</span>
         <span class="label">{{ site.data.theme.t.subscribe | default: 'Subscribe' }}</span>

_includes/head.html

@@ -3,7 +3,7 @@
   <meta http-equiv="X-UA-Compatible" content="IE=edge">
   <meta name="viewport" content="width=device-width, initial-scale=1">
 
-  {% if site.gems contains 'jekyll-seo-tag' %}
+  {% if site.plugins contains 'jekyll-seo-tag' or site.gems contains 'jekyll-seo-tag' %}
     {% comment %}
       Add metadata for search engines and social networks if jekyll-seo-tag plugin is enabled
     {% endcomment %}
@@ -26,7 +26,7 @@
     <link rel="stylesheet" href="https://fonts.googleapis.com/css?family={% for font in site.data.theme.google_fonts %}{{ font.name | replace: ' ', '+' }}{% if font.weights %}:{% endif %}{{ font.weights | remove: ' ' }}{% if forloop.last != true %}|{% endif %}{% endfor %}">
   {% endif %}
 
-  {% if site.gems contains 'jekyll-feed' %}
+  {% if site.plugins contains 'jekyll-feed' or site.gems contains 'jekyll-feed'%}
     {% comment %}
       Add Atom feed link if jekyll-feed plugin is enabled
     {% endcomment %}

_layouts/home.html

@@ -13,7 +13,7 @@
         <h2>{{ site.data.theme.t.posts | default: 'Posts' }}{% if paginator.page > 1 %}{{ site.data.theme.t.page | default: 'Page' | prepend: ' - ' | append: ' ' }}{{ paginator.page }} {{ site.data.theme.t.of | default: 'of' }} {{ paginator.total_pages }}{% endif %}</h2>
       </header>
       <div class="entries">
-        {% if site.plugins contains 'jekyll-paginate' and page.paginate %}
+        {% if site.plugins contains 'jekyll-paginate' and page.paginate or site.gems contains 'jekyll-paginate' and page.paginate %}
           {% comment %}
             Add paginator.posts loop if jekyll-paginate plugin is enabled
             and page.paginate == true