Attempt to accelerate build with Homebrew

.travis.yml

@@ -5,6 +5,9 @@ dist: bionic
 cache:
   directories:
   - vst/download/
+  # macOS Homebrew
+  - $HOME/Library/Caches/Homebrew
+  - /usr/local/Homebrew
 
 jobs:
   include:
@@ -48,6 +51,11 @@ jobs:
     install: .travis/install_osx.sh
     script: .travis/script_osx.sh
     after_success: .travis/prepare_osx.sh
+    before_cache:
+      - brew cleanup
+      # Credit https://discourse.brew.sh/t/best-practice-for-homebrew-on-travis-brew-update-is-5min-to-build-time/5215/9
+      # Cache only .git files under "/usr/local/Homebrew" so "brew update" does not take 5min every build
+      - find /usr/local/Homebrew \! -regex ".+\.git.+" -delete
 
   - name: "MOD devices arm"
     env: