Travis: plain pip install of matplotlib
Travis: plain pip install of matplotlib

Do not need to upgrage or ignore installed because there is no
pre-installed matplotlib on the travis worked node.

wizmer/NeuroM,liesbethvanherpe/NeuroM,mgeplf/NeuroM,eleftherioszisis/NeuroM,BlueBrain/NeuroM,lidakanari/NeuroM,juanchopanza/NeuroM