terrain-background only shows to zoom level 7

[dasheww]

When using the URL to access the terrain-background data, zoom level 7 is the highest resolution available. When using terrain, zoom level 13 can be accessed:

http://tile.stamen.com/terrain/{z}/{x}/{y}.jpg
http://tile.stamen.com/terrain-background/{z}/{x}/{y}.jpg

Request terrain-background have the same zoom level as terrain