Update Python requirement in INSTALL

PyXML has not been required to build the policy and its documentation
since at least Python 2.6, which comes with an "xml" module.

Moreover, some support scripts requires Python 2.6 or above (and are
compatible with Python 3.4, maybe also with other versions of Python 3).
Add the minimum supported version of Python in INSTALL.

ML thread: http://oss.tresys.com/pipermail/refpolicy/2014-November/007440.html