Updating tasks App Engine sample.

Reviewed in https://codereview.appspot.com/6973047/
diff --git a/samples/tasks_appengine/app.yaml b/samples/tasks_appengine/app.yaml
index 27372f7..86aae97 100644
--- a/samples/tasks_appengine/app.yaml
+++ b/samples/tasks_appengine/app.yaml
@@ -1,11 +1,16 @@
 application: mytasks
 version: 1
-runtime: python
+runtime: python27
 api_version: 1
+threadsafe: true
 
 handlers:
 - url: /css
   static_dir: css
 
 - url: .*
-  script: main.py
+  script: main.application
+
+libraries:
+- name: jinja2
+  version: latest