Revert pannellum to 2.4.1 and add to greenkeeper ignore

Seems they don't publish the built files anymore?

package.json

@@ -48,7 +48,7 @@
     "martinez-polygon-clipping": "0.5.0",
     "node-diff3": "1.0.0",
     "osm-auth": "1.0.2",
-    "pannellum": "2.5.1",
+    "pannellum": "2.4.1",
     "q": "1.5.1",
     "rbush": "2.0.2",
     "which-polygon": "2.2.0",
@@ -102,7 +102,7 @@
   },
   "greenkeeper": {
     "label": "chore-greenkeeper",
-    "ignore": []
+    "ignore": ["pannellum"]
   },
   "engines": {
     "node": ">=6.0.0",