Make sure we have an old and working py.test installed, as a temporary measure
Only upgrade numpy -- everything else should use the specified versions.
Merge pull request #1501 from mdboom/downgrade-pytest

Attempt to fix Travis after pytest 2.4.0 upgrade
Split the SIPModel class into forward and inverse model. Corrected inverse of ShiftModel and ScaleModel. Fixed name in parameter lookup.
ensure rotations work with scalar input.
use docstrings from base class
pep8
correct typo
Add Ring2DModel to __all__