Update package.json to specify version 0.5.1

The current noVNC release is 0.5.1, but package.json still
says version 0.5.0.  This fixes that.

Fixes #441

package.json

 {
   "name": "noVNC",
-  "version": "0.5.0",
+  "version": "0.5.1",
   "description": "An HTML5 VNC client",
   "main": "karma.conf.js",
   "directories": {