fix ipv6 support for geoip

Simple fix to support the IPv6 databases in the filter.

Fixes 
- https://logstash.jira.com/browse/LOGSTASH-1100

and partly
- https://logstash.jira.com/browse/LOGSTASH-1124
- https://logstash.jira.com/browse/LOGSTASH-2163

reworked from https://github.com/elasticsearch/logstash/pull/1350