EBS Auditor: Issues are not removed on volume deletion

[bunjiboys]

When a volume which has an associated issue is deleted, before being fixed, the issue is not correctly removed, but instead causes the auditor to crash with a NoneType exception

Traceback (most recent call last):
  File "/opt/pyenv/lib/python3.5/site-packages/cinq_scheduler_sqs/__init__.py", line 348, in execute_worker
    worker.run()
  File "/opt/pyenv/lib/python3.5/site-packages/cinq_auditor_ebs/__init__.py", line 46, in run
    data = self.update_data()
  File "/opt/pyenv/lib/python3.5/site-packages/cinq_auditor_ebs/__init__.py", line 71, in update_data
    fixed_issues = self.process_fixed_issues(volumes, existing_issues)
  File "/opt/pyenv/lib/python3.5/site-packages/cinq_auditor_ebs/__init__.py", line 175, in process_fixed_issues
    fixed_issues.setdefault(issue.volume.account, []).append(issue)
AttributeError: 'NoneType' object has no attribute 'account'

To avoid this, we need to check if the issue.volume reference is None during the cleanup phase, as this will only be None if the resource it references doesn't exist, ie. the volume has been deleted

[laf-rge]

This is probably related to #112