Support setting a custom Python version

[TheLastProject]

No idea of this will work, just an attempt at #3.

[TheAssassin]

I did conda install python="$CONDA_VERSION", which probably works as well as your script. There is a problem however that neither option allows for installing any Python 3 version but only the latest stable, at least in my tests. We should find a solution for that. If you think this is good enough, though, we can merge this as-is, and look into this later.

[TheLastProject]

Requesting Python 3.4 seems to work just fine: https://travis-ci.org/Pext/Pext/jobs/440934890

And my build fails here because SIP needs Python 3.5 at least.

Want me to make another commit to request 3.5 instead?

[TheLastProject]

Hmm... do we actually copy the right virtualenv though? I find the code a bit confusing at some points.

[TheLastProject]

Okay, tried switching to conda install as well. Maybe you just did it too late into the process? Works fine here.

[TheLastProject]

Forgot to post the log: https://travis-ci.org/Pext/Pext/jobs/440974701

[TheAssassin]

Okay, seems to work. Let's merge it.

[TheAssassin]

Thanks!