Make sprite file publicly available

[leonhandreke]

https://github.com/benjamintd/mapbox-maps/blob/dfda321/src/styles/style.json specifies "sprite": "mapbox://sprites/benjamintd/cj2wkuwc000582spd6gz9cf00". This file doesn't seem to be accessible without your access token. This issue doesn't seem to exist with the glyphs file just below, it seems to have more liberal permissions.

Adding this file to the to the source distribution would probably be the best option here, but it's probably sufficient for now to just change the permissions to make it accessible to everyone.