Try to work around build failure on Travis

I can reproduce that failure locally, using tox.  Only python 2.7 fails,
Python 2.6 and 3.3 (and PyPy) are fine.  The failure is a difference in
filenames (relative versus absolute) in test_sa4.html.  I've tried
digging, but gave up.

The failure appears if I run tox -e py27.  It doesn't appear if I run
tox -e py27 --develop.  So let's tell Travis to use the latter, to at
least catch new regressions.