Add pep8, pyflakes wheel hash to requirements

Also pins pep8 at 1.5.7.

PyPI will send back wheels or tarballs, and there is no way to know
which will come back when installing a package through pip.

Fixes bug 1047020
Merge pull request #2263 from bramwelt/add-wheel-hashes-to-requirements

fix bug 1047020 - Add Wheel Hashes to Requirements