Tweak Travis config to run image tests.
Tweak Travis config to run image tests.

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