update DarkGray basemap info for new production servers

updated urls, subdomains and zoomLevels for DarkGray basemap and labels, as it has now left beta
grunt tests passing.

http://www.arcgis.com/home/item.html?id=1970c1995b8f44749f4b9b6e81b5ba45
http://services.arcgisonline.com/arcgis/rest/services/Canvas/World_Dark_Gray_Base/MapServer