According to https://formulae.brew.sh/formula/protobuf, @3.6 is the oldest version available

Author: kripton

.travis.yml

@@ -324,8 +324,8 @@ before_install:
  - if [ "$TRAVIS_OS_NAME" == "osx" -a "$LIBFTDI" != "1" ]; then brew install libftdi0; fi # install libftdi0
  - if [ "$TRAVIS_OS_NAME" == "osx" -a "$LIBFTDI" == "1" ]; then brew install libftdi; fi # install the latest libftdi
  - if [ "$TRAVIS_OS_NAME" == "osx" -a "$PROTOBUF" == "latest" ]; then brew install protobuf; fi
- - if [ "$TRAVIS_OS_NAME" == "osx" -a "$PROTOBUF" != "latest" ]; then brew install protobuf@3.1; fi
- - if [ "$TRAVIS_OS_NAME" == "osx" -a "$PROTOBUF" != "latest" ]; then brew link -f protobuf@3.1; export PKG_CONFIG_PATH=/usr/local/opt/protobuf@3.1/lib/pkgconfig; brew install --build-from-source --ignore-dependencies --env=std protobuf-c; fi # When protobuf is not on the latest release
+ - if [ "$TRAVIS_OS_NAME" == "osx" -a "$PROTOBUF" != "latest" ]; then brew install protobuf@3.6; fi
+ - if [ "$TRAVIS_OS_NAME" == "osx" -a "$PROTOBUF" != "latest" ]; then brew link -f protobuf@3.6; export PKG_CONFIG_PATH=/usr/local/opt/protobuf@3.6/lib/pkgconfig; brew install --build-from-source --ignore-dependencies --env=std protobuf-c; fi # When protobuf is not on the latest release
  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then mkdir -p ${HOME}/Library/Python/2.7/lib/python/site-packages; echo 'import site; site.addsitedir("/usr/local/lib/python2.7/site-packages")' >> ${HOME}/Library/Python/2.7/lib/python/site-packages/homebrew.pth; fi
  - if [ "$TRAVIS_OS_NAME" == "osx" -a "$CPPUNIT" != "1.14" ]; then brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/e6e43cf6a3%5E/Formula/cppunit.rb; fi # install a slightly older cppunit, as latest needs C++11 support
  - if [ "$TRAVIS_OS_NAME" == "osx" -a "$CPPUNIT" == "1.14" ]; then brew install cppunit; fi # install the latest cppunit, which needs C++11