MNT: Make 3.6 main build version on Travis
MNT: Make 3.6 main build version on Travis

Drop 3.5 back to just a single test run.

ahaberlie/MetPy,dopplershift/MetPy,jrleeman/MetPy,Unidata/MetPy,ShawnMurd/MetPy,dopplershift/MetPy,jrleeman/MetPy,Unidata/MetPy,ahaberlie/MetPy