fix osm2vectortiles/export error : undefined symbol
fix osm2vectortiles/export error : undefined symbol

fix error message in travis:
    "node: symbol lookup error: /usr/local/lib/node_modules/mapnik/lib/binding/node-v46-linux-x64/mapnik/input/postgis.input: undefined symbol: _ZN6icu_5513UnicodeStringC1ERKS0_"
osm2vectortiles/osm2vectortiles,osm2vectortiles/osm2vectortiles,geometalab/osm2vectortiles,geometalab/osm2vectortiles