fix: use correct signal-cli default port 8080 and robust connectivity check

- Change default noise-signal-cli-address to port 8080 (signal-cli's --http default)
- Replace getVersion (not a valid JSON-RPC method) with noisePing probe
- noise-rpc-check now returns t on any valid JSON-RPC response
- Update README to reflect correct default port

README.org

@@ -14,7 +14,7 @@ The project assumes a working installation of signal-cli and interfaces with tha
 
 For SQLite access we use this library - https://github.com/pekingduck/emacs-sqlite3-api
 
-We communicate with signal-cli via JSON-RPC over HTTP on localhost port 9128 by default but this can be configured by specifying =noise-signal-cli-address=
+We communicate with signal-cli via JSON-RPC over HTTP on localhost port 8080 by default but this can be configured by specifying =noise-signal-cli-address=
 
 ** User Experience
 
@@ -61,7 +61,7 @@ Below are recipes for several popular Emacs package managers.
              :protocol https)
     :config
     ;; Optional: set a custom signal-cli address
-    ;; (setq noise-signal-cli-address "http://localhost:9128")
+    ;; (setq noise-signal-cli-address "http://localhost:8080")
 
     ;; Start Noise automatically
     (noise-mode 1))