roles/graylog: Update Graylog repository RPM URL

Graylog 3.3 is currently installed on logs0.  Attempting to install the
*graylog-3.1-repository* package causes a transaction conflict, making
the task and playbook fail.

roles/graylog/tasks/main.yml

@@ -1,6 +1,6 @@
 - name: ensure graylog repository is available
   package:
-    name=https://packages.graylog2.org/repo/packages/graylog-3.1-repository_latest.rpm
+    name=https://packages.graylog2.org/repo/packages/graylog-3.3-repository_latest.rpm
     state=present
   tags:
   - install