GeoIP: switch to GEOIP_MEMORY_CACHE from GEOIP_INDEX_CACHE

Using GEOIP_INDEX_CACHE on some older versions of libGeoIP (e.g. 1.5.0
which is the default version on CentOS 7) leads to "Error reading file"
error while opening completely valid GeoIP.dat:

    # cat test.c
    #include <stdio.h>
    #include "GeoIP.h"

    int main(void) {
      GeoIP *g;

      g = GeoIP_open("/tmp/GeoIP.dat", GEOIP_INDEX_CACHE);

      if (g == NULL) {
        printf("error!\n");
      }

      GeoIP_delete(g);

      exit(0);
    }
    # cc -lGeoIP -o test test.c
    # ./test
    Error reading file /tmp/GeoIP.dat
    error!
    # sed -i -e 's,GEOIP_INDEX_CACHE,GEOIP_MEMORY_CACHE,' test.c
    # cc -lGeoIP -o test test.c
    # ./test
    # geoiplookup -f /tmp/GeoIP.dat -v 8.8.8.8
    GeoIP Country Edition: GEO-106FREE 20180327 Build 1 Copyright (c) 2018 MaxMind Inc All Rights Reserved

Also tested with recent GeoLite databases converted from new format
into legacy format, distributed here:

    https://mailfud.org/geoip-legacy/

Author: defanator

src/utils/geo_lookup.cc

@@ -70,7 +70,7 @@ bool GeoLookup::setDataBase(const std::string& filePath,
 
 #ifdef WITH_GEOIP
     if (m_version == NOT_LOADED) {
-        m_gi = GeoIP_open(filePath.c_str(), GEOIP_INDEX_CACHE);
+        m_gi = GeoIP_open(filePath.c_str(), GEOIP_MEMORY_CACHE);
         if (m_gi == NULL) {
             intGeo.append("GeoIP: Can't open: " + filePath + ".");
         } else {