Add v6.0 to dropdown menu

Author: toshimaru

src/_layouts/default.html

@@ -34,7 +34,8 @@
             v{{ page.version }}
           </a>
           <div class="dropdown-menu dropdown-menu-md-right" aria-labelledby="dropdownMenuLink">
-            <a class="dropdown-item" href="/">v6.0</a>
+            <a class="dropdown-item" href="/">v6.1</a>
+            <a class="dropdown-item" href="/6.0/">v6.0</a>
             <a class="dropdown-item" href="/5.2/">v5.2</a>
           </div>
         </div>