cmd/present: only include play.js in article when necessary

There is no need to add play.js in article page when
playground is disabled.

Signed-off-by: Tw <tw19881113@gmail.com>

Change-Id: If40d256c89d7bd37efff9e08bb176bd3ea2d6f88
Reviewed-on: https://go-review.googlesource.com/16415
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/present/templates/article.tmpl

@@ -56,7 +56,11 @@
         {{end}}
       </div>
     </div>
+
+    {{if .PlayEnabled}}
     <script src='/play.js'></script>
+    {{end}}
+
     <script>
       (function() {
         // Load Google Analytics tracking code on production site only.