travis: fix macOS 10.13 build

homebrew is removing 10.13 support and some of the dependencies start
building rom source now. we will just pin the last working homebrew
version, similar to the 10.12 build

.travis.yml

@@ -37,12 +37,17 @@ matrix:
       osx_image: xcode11.6
     - <<: *mac
       osx_image: xcode10.1
+      env:
+        - HOMEBREW_NO_AUTO_UPDATE=1
+        - HOMEBREW_NO_INSTALL_CLEANUP=1
+        - CI_HOMEBREW_HASH=7242872d7878f1a4c2706e5837faafcf0782b58d
     - <<: *mac
       osx_image: xcode9.2
       env:
         - HOMEBREW_NO_AUTO_UPDATE=1
         - HOMEBREW_NO_INSTALL_CLEANUP=1
         - CI_SWIFT_FLAGS="\-target x86_64-apple-macosx10.12"
+        - CI_HOMEBREW_HASH=55e02323812604add9a69bab8730319b9255a697
     - os: freebsd
       compiler: clang
     - os: linux
@@ -128,13 +133,13 @@ before_install:
                 brew untap caskroom/cask
             fi
             brew update
-            if [[ "$TRAVIS_OSX_IMAGE" == "xcode9.2" ]]; then
+            if [[ -n "$CI_HOMEBREW_HASH" ]]; then
                 pushd "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core"
-                git checkout --force 55e02323812604add9a69bab8730319b9255a697
+                git checkout --force $CI_HOMEBREW_HASH
             fi
             brew install $install
             brew link --overwrite python
-            if [[ "$TRAVIS_OSX_IMAGE" == "xcode9.2" ]]; then
+            if [[ -n "$CI_HOMEBREW_HASH" ]]; then
                 git checkout master
                 popd
             fi