Stop using local ZODB snapshot tarball

Also relax version pins (we don't want outdated alphas if actual
releases are out already).
Use a different test request to make it work on Py3.3

zope.publisher.tests.httprequest does not work on Python 3: it wants
StringIO.StringIO.
Make .travis.yml use tox

Should make the tests pass on Python 3.3 on travis (those minimum
version number constraints in tox.ini matter now that pip doesn't
install pre-releases by default).