Test on Python 3.4, use tox on Travis

Python 3 builds on Travis failed due to an old zope.site.  Oh, now I get
it!  tox uses pre-releases by default!  So a regular pip install would
fetch the last stable zope.site (3.9.2) and fail on Python 3, while pip
install --pre in tox would fetch zope.site 4.0.0a1 and succeed.