update_files: add MANIFEST.in lines for versioneer.py and _version.py

This replaces the manual instructions to update MANIFEST.in, since it's
easy to miss, and the consequences of leaving versioneer.py out of
MANIFEST.in is that things work fine from a source checkout, but not
from a 'setup.py sdist' -generated tarball, which generally breaks pip.
Addresses issue #20.