Version bump because of missing icons

The webextension icons were missing in the Mozilla addons upload. We
need a newer version to update it.

proxy/webext/manifest.json

 {
 	"manifest_version": 2,
 	"name": "Snowflake",
-	"version": "0.0.3",
+	"version": "0.0.4",
 	"description": "Snowflake is a WebRTC pluggable transport for Tor.",
 	"background": {
 		"scripts": ["snowflake.js"],