Use SPDX license expression syntax. Fixes #419.

bower.json

@@ -2,7 +2,7 @@
   "name": "converse.js",
   "description": "Web-based XMPP/Jabber chat client written in javascript",
   "version": "0.9.3",
-  "license": "MPL",
+  "license": "MPL-2.0",
   "devDependencies": {
     "jasmine": "https://github.com/jcbrand/jasmine.git#1_3_x",
     "otr": "0.2.12",