Bump the Xcode minimum version to 5.1

Upstream Chromium hasn't been building with the version of clang
available in Xcode 5.0 for a while and we have to disable the build
until we see a benefit to maintain this build environment.

Change-Id: Ia91844dc7f3be1b1623610be417409d0d065c623
Reviewed-by: Michael Bruning <michael.bruning@digia.com>