Travis CI build improvements

Hello,

I noticed the Travis CI configuration could use some work. I suggest
taking a look at <https://github.com/travis-perl/helpers> since it can
automatically install all the dependencies and send the coverage to
Coveralls.

We could additionally look at automatically building on Windows (using
Appveyor) and macOS (using Travis CI) as well.

I helped the PDL project improve their builds and coverage in the past
using the same tools.

Regards,
- Zakariyya Mughal