Make it possible to run the test suite with distutils

This allows to run "python setup.py test", or (optionally) a coverage
analysis with "coverage run setup.py test".
Do not use Green to run tests on Travis-CI

Python <2.7 is not supported by the Green test runner, so make Travis-CI
use coverage and distutils directly to run the test suite now that Python
2.6 is in the list of versions to build with.