Travis - Cache Homebrew downloaded bottles to speedup macOS builds

[skip appveyor]

.travis.yml

@@ -7,6 +7,7 @@ language: cpp
 
 cache:
   directories:
+  - $HOME/Library/Caches/Homebrew
   - qwt
   - D2XX
 
@@ -50,5 +51,8 @@ before_script:
 script:
 - travis/$TRAVIS_OS_NAME/script.sh
 
+before_cache:
+- if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew cleanup; fi
+
 after_success:
 - travis/$TRAVIS_OS_NAME/after_success.sh