Cache for Homebrew. [skip appveyor]

.travis.yml

@@ -30,12 +30,25 @@ jobs:
     - DEPLOY=true
     - LEGACY=false
     osx_image: xcode11
+    cache:
+      directories:
+      - $HOME/Library/Caches/Homebrew
+      - /usr/local/Homebrew
   - os: osx
     compiler: clang
     env:
     - DEPLOY=true
     - LEGACY=true
     osx_image: xcode9.2
+    cache:
+      directories:
+      - $HOME/Library/Caches/Homebrew
+      - /usr/local/Homebrew
+before_cache:
+  - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then brew cleanup; fi
+# 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
+  - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then find /usr/local/Homebrew \! -regex ".+\.git.+" -delete; fi
 before_install:
 - |
   if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
@@ -63,6 +76,7 @@ before_install:
     else
       unset SSL_CERT_FILE;
       brew tap-new ${USER}/local-qt;
+      brew update > /dev/null;
       brew extract --version 5.11 qt ${USER}/local-qt; 
       brew install ${USER}/local-qt/qt@5.11;
     fi