os_version == 10.13.5 # Oldest CI nodes are currently 10.13.3
brew cmake >= 3.11.2 # LLVM docs state this is the minimum version required.
brew ninja >= 1.8.2 # Stage2 verbose output
pip psutil >= 5.4.5 # Required for timeouts in the GTest Suite.
pip six >= 1.11.0 # Required for urllib
xcode == 9.4.1