use tuple comparison for version info since on py2.6 this is an unnamed tuple

This fixes issue #178.
In python tuples are compared element by element so the first element (the major version number) is the only one that matters. 
I also moved to >=3 because I think this is more readable, but that is totally a matter of taste :)

I would recommend setting up a [travis CI](https://travis-ci.org/) job that tries to install autobahn on all available python versions. If you're interested I can do most of the work, but you'd have to activate the GitHub service hook.

Thanks for this great library, we're long time users here at work!