travis: Skip nfs-common installation
    
    Installing nfs-common is broken on trusty since build #807
    
      https://travis-ci.org/libvirt/libvirt/builds/326705054
    
    It's probably a transient error on Travis' side, so just comment
    it out for the time being to allow builds to proceed.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Daniel P. Berrange <berrange@redhat.com>