fix(wx): fix version format in manifest

the syntax defined for Chrome's version is more restrictive than that
used by Firefox:
- https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json/version
- https://developer.chrome.com/extensions/manifest/version

Part of #218 effort

add-on/manifest.json

@@ -2,7 +2,7 @@
     "manifest_version": 2,
     "name": "IPFS Companion (Development Channel)",
     "short_name": "IPFS Companion",
-    "version" : "2.0.0alpha5",
+    "version" : "2.0.5",
 
     "description": "Access IPFS resources via custom HTTP2IPFS gateway",
     "homepage_url": "https://github.com/lidel/ipfs-firefox-addon",