Test wheel dists installation in CI

[shacharoo]

Opened due to #289.
I think this should be checked for the latest in every (supported) major python version (i.e. 2.7 and 3.6), not for the entire build matrix. @Nurdok?

[Nurdok]

I think that given our current Travis configuration it will actually be easier to just test on all Python version, so I don't see a reason to limit this. Overall, this is a great idea.