Hit /versions instead of / since it is actually a known endpoint

lib/sync.js

@@ -562,14 +562,9 @@ SyncApi.prototype._sync = function(syncOptions, attempt) {
  */
 SyncApi.prototype._pokeKeepAlive = function() {
     return this.client._http.requestWithPrefix(
-        undefined, "GET", "/", undefined,
-        undefined, httpApi.PREFIX_R0, 5 * 1000
-    ).catch(function(err) {
-        if (err.httpStatus > 0) { // we hit the server alright
-            return q();
-        }
-        throw err;
-    });
+        undefined, "GET", "/_matrix/client/versions", undefined,
+        undefined, "", 5 * 1000
+    );
 };
 
 /**