.travis.yml: Use 2.7_with_system_site_packages

Travis doesn't use system packages by default [1]:

> CI Environment uses separate virtualenv instances for each Python
> version. System Python is not used and should not be relied on. If
> you need to install Python packages, do it via pip and not apt.
>
> If you decide to use apt anyway, note that Python system packages
> only include Python 2.7 libraries on Ubuntu 12.04 LTS. This means
> that the packages installed from the repositories are not available
> in other virtualenvs even if you use the --system-site-packages
> option.

but 2.7 is fine for us, and it's annoying to install OpenCV using
other means (there's currently no Conda package).  Instead, use the
(undocumented?) _with_system_site_packages suffix mentioned by Donald
Stufft [2]:

> However there's a work around here which I actually think is a much
> nicer way of handling things.
> ...
> You can do:
>
>   language: python
>
>   python:
>     - "pypy"
>     - 2.6
>     - 2.7
>     - "2.7_with_system_site_packages"
>     - 3.2
>     - "3.2_with_system_site_packages"
>     - 3.3
>
> And get system site packages on the two Python versions it is
> supported on, and as a bonus you can test with and without system
> site packages for the same Python version.

[1]: http://docs.travis-ci.com/user/languages/python/#Travis-CI-Uses-Isolated-virtualenvs
[2]: https://github.com/travis-ci/travis-ci/issues/2219#issuecomment-41804942