Remove -I (--ignore-installed) option from pip
Use apt-get again to install several Python dependencies

Forget what we said in commit 6e2283a0523daca29d1d885da6e. Downloading
the binary packages with apt-get is tremendously faster than compiling
lots of C code with pip every time the Travis tests are run. We can do
this because, unlike in Debian GNU/Linux, the versions of the packages
available in Ubuntu 12.04 are recent enough to be used to run our unit
tests.
Test against Python 2.7 only

Although 'system_site_packages' is set to 'true' in travis.yml, the
Python 2.6 virtual environment cannot see the packages installed with
apt-get. The reason for this is that on Ubuntu 12.04 system packages
only include Python 2.7 libraries. Thanks so much to Josh Kalderimis
and Mathias Meyer, from the Travis CI support team, for spinning up
a debug virtual machine and helping me figure this out.