Vulnix

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- `.drv` files may be specified directly on the command line.
- Updated PyPI dependencies.
- Document system requirements (12).
- Don't leave large files in /tmp around.
- Remove duplicate CVEs in output (25).
- Fix bug with reporting less than 3 vulnerabilities (28).

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- Packaging improvements: pin versions in setup.py, include NVDCVE test data in
sdist.
- Reduce NVDCVE fixture size. This cuts tests run time by more than 50%.

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- Skip `/nix/var/nix/gcroots/booted-system` during system check.
- Make output a bit easier to read by removing visual clutter.

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- Improve CPU and memory usage: refactored the way we fetch, parse, store and
process data. We now leverage ZODB as the storage for parsed data that is
efficient to look up.

On our test systems this caused memory usage to drop from > 1GiB to ~70MiB
and a pure evaluation of existing data to around 7-10 seconds.

This change requires a re-retrieval of all historic sources.

- Improve unit test coverage with at least a smoke test for our new fetching
procedure.

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- Keep a reverse index: product name -> vulnerabilities to speed up scan process.
- Mark 'in progress' vulnerabilities with an asterisk
- The '-w' switch accepts URLs, too
- vulnix no longer scans /var/nix/var/gcroots/booted-system
- only cached files are saved (archives are to be deleted)
- added travis build: runs periodically against nixpkgs/master and updates
requirements*.nix files in case of success

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- Add `src` to PYTHONPATH so that tests run also on older NixOS versions
(tested on 15.09).
- Correct URL, add metadata.
- Add nix to propagatedBuildInputs, as vulnix calls `nix-store` at runtime.