drop Python 2.6 (.travis.yml, doc, setup.py)

Twisted 15 dropped support for it, which causes Travis CI tests to fail on 2.6.
We still theoretically support older versions of Twisted, so perhaps we should
configure Travis to test with those? I think we should drop Python 2.6 in any
case since distros are all on 2.7 now.

I'm leaving Travis running (and ignoring) the failing PyPy tests because I
don't know why that is there.