trying Python 3.6 since travis build hangs for Python 3.7
trying Python 3.6 since travis build hangs for Python 3.7

https://travis-ci.org/paulscherrerinstitute/pcaspy/jobs/580260173#L1000

paulscherrerinstitute/pcaspy,paulscherrerinstitute/pcaspy,paulscherrerinstitute/pcaspy