Travis: Remove testing of 1.6

* Don't support 1.6 anymore. This version is nearly 3 years old by now.
* Remove minor version numbers. This might make it harder to track
whether the error is on our side or due to a changed api of our
dependencies.
Merge pull request #36 from FRidh/travis

Travis: Remove testing of 1.6
Setup: Update dependencies
Merge pull request #35 from FRidh/setup

Setup: Update dependencies
Turbulence: Improved plot layout
Merge branch 'master' of github.com:python-acoustics/python-acoustics into turbulence
Merge pull request #40 from FRidh/turbulence

Turbulence: Improved plot layout