Travis: test IPv6-only v3 single onion services on macOS tor stable

Using networks/single-onion-v23-ipv6-md.

This test requires 0.4.1, which is available in macOS homebrew's
online package repository. (IPv6 v3 single onion services require
23588, which was merged into 0.4.1.1-alpha.)

The fix for 30928 updates homebrew in all the macOS jobs, so we
also get the latest tor version from homebrew.

Closes 30279.

.travis.yml

@@ -41,9 +41,8 @@ matrix:
       os: osx
       language: c
       python:
-    ## v2 onion services only, see #30279 for v3 onion services
-    ## needs #23588 in macOS homebrew, once 0.4.1-stable is released
-    - env: TOR="stable-release" NETWORK_FLAVOUR="single-onion-ipv6-md"
+    ## v3 onion service IPv6 tests
+    - env: TOR="stable-release" NETWORK_FLAVOUR="single-onion-v23-ipv6-md"
       os: osx
       language: c
       python: