Use `browser_specific_settings` instead of `applications`

As per MDN's documentation, `applications` is a non-standard
alias for `browser_specific_settings` in `manifest.json`:

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_specific_settings#Browser_compatibility

Best to use the official manifest key.

dist/firefox/publish-signed-beta.py

@@ -161,7 +161,7 @@ with zipfile.ZipFile(raw_xpi_filepath, 'r') as zipin:
             data = zipin.read(item.filename)
             if item.filename == 'manifest.json':
                 manifest = json.loads(bytes.decode(data))
-                manifest['applications']['gecko']['update_url'] = 'https://raw.githubusercontent.com/{0}/{1}/master/dist/firefox/updates.json'.format(github_owner, github_repo)
+                manifest['browser_specific_settings']['gecko']['update_url'] = 'https://raw.githubusercontent.com/{0}/{1}/master/dist/firefox/updates.json'.format(github_owner, github_repo)
                 data = json.dumps(manifest, indent=2, separators=(',', ': '), sort_keys=True).encode()
             zipout.writestr(item, data)
 

dist/firefox/updates.template.json

@@ -4,7 +4,7 @@
    "updates": [
     {
      "version": "$ext_version",
-     "applications": { "gecko": { "strict_min_version": "55" } },
+     "browser_specific_settings": { "gecko": { "strict_min_version": "55" } },
      "update_info_url": "https://github.com/gorhill/uBlock/releases/tag/$tag_version",
      "update_link": "https://github.com/gorhill/uBlock/releases/download/$tag_version/uBlock0_$tag_version.firefox.signed.xpi"
     }

platform/firefox/manifest.json

@@ -1,10 +1,4 @@
 {
-  "applications": {
-    "gecko": {
-      "id": "uBlock0@raymondhill.net",
-      "strict_min_version": "55.0"
-    }
-  },
   "author": "All uBlock Origin contributors",
   "background": {
     "page": "background.html"
@@ -18,6 +12,12 @@
     "default_title": "uBlock Origin",
     "default_popup": "popup.html"
   },
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "uBlock0@raymondhill.net",
+      "strict_min_version": "55.0"
+    }
+  },
   "commands": {
     "launch-element-zapper": {
       "description": "__MSG_popupTipZapper__"