Upgrade pypy to 5.8, use pre-built numpy/scipy wheels

numpy 1.13.0 fails with pypy 5.7.1, so this upgrades to 5.8.0.  I've
also uploaded pre-built .whl files to imaginary.ca (checked every 4
hours and rebuilt if needed), and list that as an extra pypi location
under the pypy pip install to avoid the long travis pypy build times for
a new release or branch.