Fix local development by keeping tmp/pids around (#1695)

If you clone the project and try to start the app, it'll fail with:

```
No such file or directory @ rb_sysopen - /Users/nickcharlton/projects/administrate/tmp/pids/server.pid (Errno::ENOENT)
```

Rails doesn't seem to be able to create this path (perhaps because it's
really running `spec/example_app`?) so let's just make sure it's there
already.