Fix batched_advisories method

[sbs2001]

The current implementation of the batched_advisories at https://github.com/nexB/vulnerablecode/blob/6609bcfdebb4c28bd30dfd9d42011323d238ee4c/vulnerabilities/data_source.py#L195 needs to be simplified

1. Don't use sets, the hash method for the advisory dataclass is broken.
2. Simplify the logic.

Eventually we want to move to non-batch proccessing so this low priority

[pombredanne]

This is all fixed and merged now.... closing!