add_the_geom_webmercator: use variable for webmercator field name
add_the_geom_webmercator: use variable for webmercator field name

... and properly quote it

Conflicts:
	lib/tasks/db_maintenance.rake

datapolitan/cartodb20,datapolitan/cartodb20,jmwenda/nationmaps,datapolitan/cartodb20,jmwenda/nationmaps,jmwenda/nationmaps,jmwenda/nationmaps,datapolitan/cartodb20