Update TravisCI tests to use the docker image

[oesteban]

Depends on #1536

[mgxd]

has this been addressed? or will in #1815?

[oesteban]

Docker images are too big for travis, I don't know if we are still interested on this...

[djarecka]

Somehow related: i have on my TODO list to check tox. I understand that once you create python environments you can use them both locally and on Travis. Do you have any thoughts on this? Would it be useful?

[mgxd]

@djarecka isn't it better to build the environment every time - that way if there is a new version of a dependency we can check to see if we can support it? also closing this issue, feel free to open if needed

[djarecka]

@mgxd - tox is used to test new version of software. My understanding is that you ask for specific Python interpreters and libraries in a similar way as in Travis file.
In my comment I wasn't clear, but I meant that you simply can easily build the environment (based on the tox file) also locally on your computer, so it might be useful for debugging I guess.
However, for now I gave up, would have to learn more how to integrate it with no python packages, and it looks like people are happy without it ;-)