Save low zoom level planet extract as merge target

.travis.yml

@@ -6,7 +6,7 @@ services:
   - docker
 before_install:
   - wget -nc -P "$TRAVIS_BUILD_DIR/import" "http://download.geofabrik.de/europe/albania-160601.osm.pbf"
-  - wget -nc -O "planet.mbtiles" -P "$TRAVIS_BUILD_DIR/export" "https://osm2vectortiles-downloads.os.zhdk.cloud.switch.ch/v2.0/planet_z0-z5.mbtiles"
+  - wget -nc -O "$TRAVIS_BUILD_DIR/export/planet.mbtiles" "https://osm2vectortiles-downloads.os.zhdk.cloud.switch.ch/v2.0/planet_z0-z5.mbtiles"
   - docker-compose pull import-external export import-osm postgis rabbitmq
   # Avoid building really expensive images
   - make fast