Enable geoip file caching

Enable geoip file caching, which increases performance by roughly 25%. I tested three scenarios:

1. No geoip lookups: ~1250 messages per second.
1. Geoip lookups with 1.4.0 setup (no caching): ~1000 messages per second.
1. Geoip lookups with caching as per this PR: ~1250 messages per second.

The fix is updating the geoip gem to 1.4.0, set option preload to true.

See issue https://logstash.jira.com/browse/LOGSTASH-2158