travis: Fix macOS build (GH-7250)

Homebrew's python is now python3, but travis preinstalls old python2.
So updated Homebrew requires `brew upgrade python` now.

This commit disables auto update and use preinstalled version of Homebrew.

.travis.yml

@@ -60,7 +60,7 @@ matrix:
       env: OPTIONAL=true
       before_install:
         # Python 3 is needed for Argument Clinic and multissl
-        - brew install xz python3
+        - HOMEBREW_NO_AUTO_UPDATE=1 brew install xz python3
         - export PATH=$(brew --prefix)/bin:$(brew --prefix)/sbin:$PATH
     - os: linux
       language: c