only run this on the app server

Made a mistake.  We need this to deploy with Capistrano.


https://github.com/javan/whenever
```markdown
Here are the basic rules:

If a server's role isn't listed in whenever_roles, it will never have jobs added to its crontab.
If a server's role is listed in the whenever_roles, then it will have all jobs added to its crontab that either list that role in their :roles arg or that don't have a :roles arg.
If a job has a :roles arg but that role isn't in the whenever_roles list, that job will not be deployed to any server.