OSX Travis builds Python not as a Framework

[nonhermitian]

Informations

• Qiskit Terra version:
• Python version:
• Operating system:

What is the current behavior?

Travis OSX builds fails when functions requiring matplotlib are imported (see #874 for example) because the default matplotlib backend does not work if Python is not built as a Framework.

Steps to reproduce the problem

What is the expected behavior?

PR's should not be held up because our Travis builds do not install Python properly for the default backend and fail.

Suggested solutions

The solution is discussed here: https://matplotlib.org/faq/osx_framework.html

Another possibility is to make a util script that changes the OSX matplotlib backend to something like TKAgg that works in non-Framework builds. The matplotlibrc is here:

import matplotlib
matplotlib.matplotlib_fname()