deprecate running EasyBuild with Python 2.6 via new check_python_version() function

[boegel]

With this change included, a deprecation warning will be printed when EasyBuild is run on top of Python 2.6:

$ python -V
2.6.9
$ eb example.eb

WARNING: Deprecated functionality, will no longer work in v5.0:
Running EasyBuild with Python 2.6 is deprecated;
see http://easybuild.readthedocs.org/en/latest/Deprecated-functionality.html for more information

...

Currently the main motivation here is to (slightly) trim down on test configurations in Travis CI, but longer term some code cleanup can be done when we've dropped support for Python 2.6.

In addition to deprecating support for Python 2.6, we also check whether the Python version being used is actually supported via the added check_python_version function.
Up until now, we were only checking the Python version in the eb wrapper, so no Python version check was done at all when EasyBuild is being used as a library...