Matplotlib: Add a test

[183amir]

This test makes sure all matplotlib dependencies are already installed
which would prevent ContinuumIO/anaconda-issues#1379 (comment)
from happening.

[ilanschnell]

Thanks for pointing this out. The problem is that when adding setuptools as a test dependency, you cannot test whether or not the package breaks if setuptools is not installed, and I always try to not list setuptools as a dependency when it is not necessary. I'll have to think about this.

[183amir]

Maybe we can run this command in the build.sh script?

[ilanschnell]

Yes! That seems like the right solution to me. Thanks!