Avoid a 301 redirect for Stamen watercolors (#299)

* Stamen only provides jpg watercolor tiles

Avoid a redirection from png to jpg for each loaded tile

* conform to travis merge rules

Travis warns "AssertionError: expected 'https://stamen-tiles-{s}.a.ssl.fastly.net/{variant}/{z}/{x}/{y}.jpg' to include '{ext}'" so let's keep the {ext} in the url as if it could change and set it to 'jpg' as an option

Author: JLuc

leaflet-providers.js

@@ -283,6 +283,7 @@
 					url: 'https://stamen-tiles-{s}.a.ssl.fastly.net/{variant}/{z}/{x}/{y}.{ext}',
 					options: {
 						variant: 'watercolor',
+						ext: 'jpg',
 						minZoom: 1,
 						maxZoom: 16
 					}