travis ci - remove glibcxx workaround check as we're building on xenial
travis ci - remove glibcxx workaround check as we're building on xenial

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