Are you sure you are using 11.04 ?. Because by default, natty is having ant 1.8. If you are using any older version of Ubuntu then you can install ant from Ubuntu repositories itself, using
sudo apt-get install ant1.8
If you have issue on 11.04, try removing ant1.7. By default, ant in 11.04 is 1.8.1. Otherwise if you specifically need ant1.8 from that ppa, you can remove your existing ant, ant1.7 and install it.