Bump lastUpgradeCheck to Xcode 9.4.
Bump lastUpgradeCheck to Xcode 9.4.

This should prevent the `Update to recommended settings` warnings when
using Xcode 9.4 and below.

PiperOrigin-RevId: 204973286

pinterest/tulsi,bazelbuild/tulsi,pinterest/tulsi,bazelbuild/tulsi,bazelbuild/tulsi,bazelbuild/tulsi,pinterest/tulsi,bazelbuild/tulsi,pinterest/tulsi,bazelbuild/tulsi,pinterest/tulsi,pinterest/tulsi