Change the way TravisCI is used.

[mpasternak]

Instead of using single python version for all builds on Travis and then using tox to setup different interpreters in different environments, setup TravisCI integration in a way such, that it uses its own mechanisms to handle Python installation and then runs tox with only a single environment (single python version) inside the build.

This way you won’t get errors about missing Python interpreter from TravisCI, that you had chances to get in the previous configuration.

[browniebroke]

I wanted to use this cookie cutter to generate a new package and found this PR. I'm interested in it because it's adding Python 3.6 to Travis 👍 I think it would make sense to update the README to states that and also the classifiers in setup.py, right?

[mpasternak]

@browniebroke , if you want to submit some pull requests, I'm willing to add them to my branch. I plan to add some more stuff to my branch, I already added "github_project_slug" because I'd rather use dashes than underscores and I want to have it different; I plan adding templates for appveryor.com (Windows) builders... I also thought a bit about adding MacOS X support to TravisCI files, so there is still much stuff to do.

[pydanny]

Over time we've organically moved to something that isn't far off from this PR. I may cherry-pick portions of @mpasternak's effort into the main branch, his Makefile enhancements in particular look very useful.