test errors on python3, matplotlib 2.0.2

[tomr-stargazer]

Hi all,

I ran into the following test errors on a fresh install of corner today:

https://gist.github.com/tomr-stargazer/b2271fda034533d084f6db94f163fdc9

relevant software versions:
nosetests --version
nosetests version 1.3.7

python -c 'import matplotlib; print(matplotlib.version)'
2.0.2

python --version
Python 3.6.0 :: Anaconda custom (x86_64)

[tomr-stargazer]

commit fe4216f of corner.py if that is relevant

[dfm]

The first error is because you need to install pandas :-)

For the rest of them, this happens because the reference images were generated before the release of MPL v2.0. We'll need to generate new reference images for v2.0.