Replace removal of vhost symlink with call to a2dissite.

My apache2 install created a symlink called `000-default` instead of `00-default`. Maybe this was a typo or it has actually changed. It is better to rely on the a2dissite command, which reliably disables the default vhost.
Merge pull request #1 from rjkip/master

Replace removal of vhost symlink with call to a2dissite.