bump leaflet version in the htmlwidgets yaml

Author: schloerke

inst/htmlwidgets/leaflet.yaml

@@ -4,7 +4,7 @@ dependencies:
     src: "htmlwidgets/lib/jquery"
     script: jquery.min.js
   - name: leaflet
-    version: 1.2.0
+    version: 1.3.1
     src: "htmlwidgets/lib/leaflet"
     script: leaflet.js
     stylesheet: leaflet.css