travis: move variables not relevant to cache contents out of env
travis: move variables not relevant to cache contents out of env

(so that changing these later doesn't invalidate the cache)

tomhughes/mapnik,naturalatlas/mapnik,mapnik/mapnik,lightmare/mapnik,tomhughes/mapnik,lightmare/mapnik,mapnik/mapnik,lightmare/mapnik,mapnik/mapnik,tomhughes/mapnik,mapnik/mapnik,naturalatlas/mapnik,naturalatlas/mapnik,naturalatlas/mapnik,lightmare/mapnik,tomhughes/mapnik