Vagrant docs: add purging the cache step

[ariarijp]

I got an error with Vagrant setup procedure in Setting up development environment (using Vagrant).

This error to solve with purging the cache.

12:32:33 worker.1 | [2016-05-01 12:32:33,191: ERROR/MainProcess] Task redash.tasks.record_event[0c4fbabf-77b0-42ed-9bf6-52f071f15d82] raised unexpected: KeyError('org_id',)
12:32:33 worker.1 | Traceback (most recent call last):
12:32:33 worker.1 |   File "/usr/local/lib/python2.7/dist-packages/celery/app/trace.py", line 240, in trace_task
12:32:33 worker.1 |     R = retval = fun(*args, **kwargs)
12:32:33 worker.1 |   File "/opt/redash/current/redash/tasks/base.py", line 13, in __call__
12:32:33 worker.1 |     return super(BaseTask, self).__call__(*args, **kwargs)
12:32:33 worker.1 |   File "/usr/local/lib/python2.7/dist-packages/celery/app/trace.py", line 437, in __protected_call__
12:32:33 worker.1 |     return self.run(*args, **kwargs)
12:32:33 worker.1 |   File "/opt/redash/current/redash/tasks/general.py", line 13, in record_event
12:32:33 worker.1 |     models.Event.record(event)
12:32:33 worker.1 |   File "/opt/redash/current/redash/models.py", line 1046, in record
12:32:33 worker.1 |     org = event.pop('org_id')
12:32:33 worker.1 | KeyError: 'org_id'

Please review my changes.

Thanks,

[arikfr]

Thanks.