Work around protobuf 2.6.1 install failure on Python 2.6.x.
Merge pull request #284 from tseaver/work_around_protobuf_2.6.1-py26

Work around protobuf 2.6.1 install failure on Python 2.6.x.
Make 'tox -e docs' rebulid docs from scratch.

Avoid make-like optimizations which can get out of sync after git operations.