Do not use $DISPLAY variable backend for matplotlib
Do not use $DISPLAY variable backend for matplotlib

This is better for both running on an external server
without having X-forwarding, as well as for TRAVIS

nyee/RMG-Py,nickvandewiele/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py,pierrelb/RMG-Py