Update README

As requested on #powerdns:
[20|11:43:49] < ion> Execute 'bundler install' on the directory. It should install the
[20|11:44:03] < cmouse> yes, if it's wrong and you know better, please do tell =)
[20|11:44:05] < cmouse> or even make an issue =)
[20|11:44:15] < ion> s/bundler/bundle/