Bump default Xcode version to 11.1
Bump default Xcode version to 11.1

Xcode 11.0 still causes an expired certificate error on some machines.
bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration