Update Travis to test with minimum requirements.
Update Travis to test with minimum requirements.

Unidata/MetPy,deeplycloudy/MetPy,jrleeman/MetPy,ahaberlie/MetPy,dopplershift/MetPy,ahill818/MetPy,ShawnMurd/MetPy,ahaberlie/MetPy,jrleeman/MetPy,Unidata/MetPy,dopplershift/MetPy