Add new audit_read access vector in capability2 class

This AV has been added in 3.16 in commit
3a101b8de0d39403b2c7e5c23fd0b005668acf48
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
Merge upstream