Update bower.json

Included the "main" property as bower is moaning about it's absence.

see -> https://github.com/bower/bower.json-spec#main

bower.json

@@ -1,6 +1,7 @@
 {
   "name": "vis",
   "version": "3.2.0",
+  "main": "dist/vis.js",
   "description": "A dynamic, browser-based visualization library.",
   "homepage": "http://visjs.org/",
   "repository": {