Update to April GeoIP2 database

My geoip-2018-04-03 branch contains the updated geoip and geoip6 files with IPv4 and IPv6 ranges and is supposed to be merged into maint-0.2.5 and other branches that are still maintained.

(Note to self when I use this ticket as template for next month: 0.2.5 is end of life on 1 May 2018, and 0.3.1 is end of life on 1 July 2018.)

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