Rework our Travis set-up.
Rework our Travis set-up.

- Build wheels for new versions as needed and get/store in S3.
- Refactor build matrix to be a bit cleaner
- Only install pandoc for doc build
- Use the 'test' extra defined in setup.py to get dependencies.

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