spec file: do not include BuildRequires for lint by default
    
    Lint is never executed from rpmbuild, so the BuildRequires for lint are
    purely informational.
    
    Include them only if %with_lint RPM macro is specified.
    
    Update .travis.yml accordingly.
    
    https://fedorahosted.org/freeipa/ticket/6418
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>