message: fix macos->osx for travis script
diff: @@ -6,7 +6,7 @@ then
  AWS_ACCESS_KEY_ID=${GOOGLE_ACCESS_KEY_ID} AWS_SECRET_ACCESS_KEY=${GOOGLE_SECRET_ACCESS_KEY} aws s3 --endpoint-url https://storage.googleapis.com cp s3://cesium-unreal-engine/2021-03-16/Launcher.zip .
  7z x Launcher.zip "-oC:\Program Files (x86)\Epic Games"
  rm Launcher.zip
  -elif [[ $TRAVIS_OS_NAME == "macos" && ! -d "/User/Shared/Epic Games/UE_4.26" ]]
  +elif [[ $TRAVIS_OS_NAME == "osx"]]
  then
  gsutil cp gs://cesium-unreal-engine/macos-2021-04-13/UE_4.26.tar.gz UE_4.26.tar.gz
  tar xfz UE_4.26.tar.gz -C $HOME
  