add new Min-replacement pattern to compute_cdf

If you don't do this you might get alternate results

https://travis-ci.org/smichr/sympy/jobs/19850479 shows the failure in
a run where 3 of the 4 python versions passed but one failed.

I have not seen this very often; I don't know why it happened in the failing case; maybe because sometimes the 3-arg Min reduces to the 2-arg case?