matplotlib version problem

[lamping7]

We're blindly upgrading packages with pip install --upgrade. I'm not particularly happy with this, but because versions change without testing, but it also helps us find problems like the current one.

With the current tag of the ros-melodic image we have the build failing with:

Downloading https://files.pythonhosted.org/packages/51/fe/84ab101f8ab543d89b6a128326f62adcdafd2781ab8362a737e6ce78eea7/matplotlib-3.1.0.tar.gz (37.2MB)
Complete output from command python setup.py egg_info:
Beginning with Matplotlib 3.1, Python 3.6 or above is required.

I'm not sure why we have the --upgrade flag specified. The PR that introduced this has no motivation for it. We have a couple options to fix this. #122. Possibly:

1. remove --upgrade
2. make a requirements.txt to lock down what we know works