Update geoip.rb for usage with Hash-Fields

If your ip-field is in a hash-field you can specify the index of the hash.
E.g.
== Json Input ==
{ "@timestamp" "2014-04-16T04:42:20+0200", "client_data" : {"name" : "Max", "ip" : "127.0.0.1"}}

== Filter Config ==
filter {
  geoip {
    source => "client_data"
    index => "ip"
    target => "geoip"
  }
}