add `no_skip` directive in unquoted_ustring rule (ref: https://github.com/mapnik/mapnik/pull/3389#issuecomment-204344223)
add `no_skip` directive in unquoted_ustring rule (ref: https://github.com/mapnik/mapnik/pull/3389#issuecomment-204344223)

naturalatlas/mapnik,CartoDB/mapnik,pnorman/mapnik,lightmare/mapnik,tomhughes/mapnik,CartoDB/mapnik,mapnik/mapnik,naturalatlas/mapnik,mapycz/mapnik,mapnik/mapnik,mapnik/mapnik,lightmare/mapnik,tomhughes/mapnik,CartoDB/mapnik,zerebubuth/mapnik,pnorman/mapnik,zerebubuth/mapnik,lightmare/mapnik,mapycz/mapnik,mapnik/mapnik,naturalatlas/mapnik,pnorman/mapnik,tomhughes/mapnik,naturalatlas/mapnik,lightmare/mapnik,tomhughes/mapnik,zerebubuth/mapnik,pnorman/mapnik,mapycz/mapnik