The use of ``python setup.py check -r -s`` is actually deprecated and
the recommended method for validating a README for PyPi upload is to
build the sdist and wheel and run ``twine check $DIST_PATH``. This
switches our validation check to use the supported method.
Change-Id: I321b8cf6f1e2116d2fe23059d5f13aa6f2ae8b4c
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>