blacklist backports.lzma v0.0.4

backports.lzma recently bumped up to v0.0.4, adding namespace support
to the package. However, they forgot to call declare_namespace() in
their __init__.py. This causes pip install failures when running

    pip install file:///docker-registry#egg=docker-registry

From inside the Dockerfile.
Merge pull request #573 from deis/hotfix-pin-lzma

blacklist backports.lzma v0.0.4