travis: make macOS builds faster

the problem here is that with time, and because the macOS VMs don't get
updated, the homebrew update is getting longer since more and more
changes have to be pulled. to prevent that, we cache the homebrew
installation folder after the update. that way we don't have to update
several months worth of updates every build. for the legacy build we
have to check put master again to actually cache the newest homebrew
version.

additionally to that, we also do the same as on the legacy build, with
the addition of not removing all installed formulas but only the ones
we don't need. so we don't need to reinstall those.

.travis.yml

@@ -1,7 +1,7 @@
 language: c
 
 _macbase:
-  - &macOld
+  - &mac
     os: osx
     compiler: clang
     env:
@@ -12,14 +12,7 @@ _macbase:
     cache:
       directories:
         - $HOME/Library/Caches/Homebrew
-  - &macNew
-    <<: *macOld
-    addons:
-      homebrew:
-        packages: ['autoconf', 'automake', 'ffmpeg', 'freetype', 'fribidi',
-                   'libass', 'libtool', 'little-cms2', 'luajit', 'nasm',
-                   'pkg-config', 'python']
-        update: true
+        - /usr/local/Homebrew
 
 _mingwbase:
   - &mingw
@@ -38,11 +31,11 @@ _mingwbase:
 
 matrix:
   include:
-    - <<: *macNew
+    - <<: *mac
       osx_image: xcode11.3
-    - <<: *macNew
+    - <<: *mac
       osx_image: xcode10.1
-    - <<: *macOld
+    - <<: *mac
       osx_image: xcode9.2
     - os: freebsd
       compiler: clang
@@ -119,25 +112,22 @@ before_install:
         fi
   - |
         if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
+            remove=$(brew list)
+            keep="gettext pcre2 git"
+            install="autoconf automake pkg-config libtool python freetype fribidi little-cms2 luajit libass ffmpeg"
+            for formula in ${keep[@]}; do remove=("${remove[@]/$formula}"); done
+            for formula in ${install[@]}; do remove=("${remove[@]/$formula}"); done
+            brew remove --force $remove --ignore-dependencies
+            brew update
             if [[ "$TRAVIS_OSX_IMAGE" == "xcode9.2" ]]; then
-                brew remove --force $(brew list) --ignore-dependencies
-                brew update
                 pushd "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core"
-                git checkout 55e02323812604add9a69bab8730319b9255a697
+                git checkout --force 55e02323812604add9a69bab8730319b9255a697
+            fi
+            brew install $install
+            brew link --overwrite python
+            if [[ "$TRAVIS_OSX_IMAGE" == "xcode9.2" ]]; then
+                git checkout master
                 popd
-                brew install autoconf
-                brew install automake
-                brew install pkg-config
-                brew install libtool
-                brew install python
-                brew install freetype
-                brew install fribidi
-                brew install little-cms2
-                brew install luajit
-                brew install libass
-                brew install ffmpeg
-            else
-                brew link --overwrite python
             fi
         fi