Drop default support for Python 2.6

[pombredanne]

The docker images have Python 2.6 by default.... this is however a version that starts to be really old and no longer supported AFAIK. Building modern CPython 2.x projects with 2.6 is likely to fail in many cases.

See my terribly bad way to work around it here: https://github.com/pombreda/thirdparty-manylinux/blob/3c35a8fa5eee12dec76993a90632e22cc6dbbe01/travis/build-wheels.sh#L8

[vanrein]

Similarly, PIP 3.4 is also run with the present images, but that has been dropped from the list of supported versions since March.