upgraded cirrus mac image (#225)

.cirrus.yml

@@ -57,22 +57,14 @@ task:
   use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true'
   name: local_tests
   osx_instance:
-    image: high-sierra-xcode-9.4.1
+    image: catalina-flutter
   env:
     PATH: $PATH:/usr/local/bin
   setup_script:
     - pod repo update
-    - git clone https://github.com/flutter/flutter.git
     - git fetch origin master
-    - export PATH=`pwd`/flutter/bin:`pwd`/flutter/bin/cache/dart-sdk/bin:$PATH
     - flutter doctor
     - pub global activate flutter_plugin_tools
-    - mkdir homebrew && curl -L https://github.com/Homebrew/brew/tarball/master | tar xz --strip 1 -C homebrew
-    - export PATH=`pwd`/homebrew/bin:$PATH
-    - brew install java
     - brew install clang-format
   build_script:
-    - export PATH=`pwd`/flutter/bin:`pwd`/flutter/bin/cache/dart-sdk/bin:$PATH
-    - export PATH=`pwd`/homebrew/bin:$PATH
-    - export PATH=`pwd`/homebrew/opt/openjdk/bin:$PATH
     - ./script/local_tests.sh