go.talks/present: Only pull in play.js if play is enabled.

R=adg
CC=golang-dev
https://golang.org/cl/7098071
diff --git a/pkg/present/parse.go b/pkg/present/parse.go
index 6fa338d..d676d6d 100644
--- a/pkg/present/parse.go
+++ b/pkg/present/parse.go
@@ -34,8 +34,9 @@
 func (d *Doc) Render(w io.Writer, t *template.Template) error {
 	data := struct {
 		*Doc
-		Template *template.Template
-	}{d, t}
+		Template    *template.Template
+		PlayEnabled bool
+	}{d, t, PlayEnabled}
 	return t.ExecuteTemplate(w, "root", data)
 }
 
diff --git a/present/templates/slides.tmpl b/present/templates/slides.tmpl
index 93d874e..d2abfa1 100644
--- a/present/templates/slides.tmpl
+++ b/present/templates/slides.tmpl
@@ -47,7 +47,9 @@
       </article>
 
   </body>
+  {{if .PlayEnabled}}
   <script src='/play.js'></script>
+  {{end}}
 </html>
 {{end}}