libre: [libretools]: disable ronn
    
    Ronn doesn't work anymore:
        $ ronn
        `RubyGems' were not loaded.
        /usr/bin/ronn:9:in `require': cannot load such file -- rubygems (LoadError)
            from /usr/bin/ronn:9:in `<main>'
    and libretools fails to build because of that.
    
    So we disable building and installing the documentation temporarily until
    ronn or rubygems is fixed.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>