Updated main field of bower.json to `["dist/vis.min.js", "dist/vis.min.css"]` (see also #256)

bower.json

@@ -1,7 +1,7 @@
 {
   "name": "vis",
   "version": "3.2.1-SNAPSHOT",
-  "main": "dist/vis.js",
+  "main": ["dist/vis.min.js", "dist/vis.min.css"],
   "description": "A dynamic, browser-based visualization library.",
   "homepage": "http://visjs.org/",
   "repository": {