.travis.yml updates

There was a recent update to TravisCI's build environment for Python. See:
http://blog.travis-ci.com/2014-04-28-upcoming-build-environment-updates/

The restructuring was such that having:

    virtualenv:
        system_site_packages: true

caused problems for Python versions which do not have a system install (e.g. anything but 2.7 and 3.3 for Ubuntu). So we had to remove the declaration and manually activate the virtual environment for 2.7 and 3.3.