Add six module as a runtime requirement
    
    Per https://lists.fedoraproject.org/pipermail/devel/2014-March/196647.html it
    turns out that "six" is a runtime requirement. Other modules, like rdflib, that
    depend on functionality will fail in a mock build where six is not installed.
    
    Signed-off-by: Dan Scott <dan@coffeecode.net>