Extract rails versions menu and cache it for the faster build (#128)

Author: toshimaru

src/_includes/rails-versions.html

@@ -0,0 +1,9 @@
+{% for rails_version in site.rails_versions %}
+    {% assign version = rails_version[0] %}
+    {% assign detail = rails_version[1] %}
+    {% if detail.latest %}
+        <a class="dropdown-item" href="/">v{{ version }}</a>
+    {% else %}
+        <a class="dropdown-item" href="/{{ version }}/">v{{ version }}</a>
+    {% endif %}
+{% endfor %}

src/_layouts/default.html

@@ -38,15 +38,7 @@
             v{{ page.version }}
           </a>
           <div class="dropdown-menu dropdown-menu-md-right" aria-labelledby="dropdownMenuLink">
-            {% for rails_version in site.rails_versions %}
-              {% assign version = rails_version[0] %}
-              {% assign detail = rails_version[1] %}
-              {% if detail.latest %}
-                <a class="dropdown-item" href="/">v{{ version }}</a>
-              {% else %}
-                <a class="dropdown-item" href="/{{ version }}/">v{{ version }}</a>
-              {% endif %}
-            {% endfor %}
+            {% include_cached rails-versions.html %}
           </div>
         </div>
       </li>