Bump windshaft version

That version contains our flavor of mapnik 3.0.15 with a bunch of
patches. See
https://github.com/CartoDB/Windshaft/blob/master/NEWS.md#version-431

package.json

@@ -47,7 +47,7 @@
     "step-profiler": "~0.3.0",
     "turbo-carto": "0.20.2",
     "underscore": "~1.6.0",
-    "windshaft": "4.2.0",
+    "windshaft": "4.3.1",
     "yargs": "~5.0.0"
   },
   "devDependencies": {