ci: update to latest xcode on Circle CI
ci: update to latest xcode on Circle CI

Xcode 9 image contains macOS 10.13 which doesn't appear to be compatible
with the latest version of Homebrew which now emits Ruby errors causing
failing master.

apiaryio/drafter,apiaryio/drafter,apiaryio/drafter,apiaryio/drafter,apiaryio/drafter