CI Fix

I fixed cython to build tag 0.20.1 rather than HEAD.  This is a temporary fix, and a more comprehensive PR  that moves the regression test dependencies into fast/nightly deps should be coming later this week.

Before: http://submit-1.batlab.org/nmi/results/details?runID=254453
After   : http://submit-1.batlab.org/nmi/results/details?runID=254458