Added hint about updating CocoaPods installation to spec

spec/unit/installer/user_project_integrator/target_integrator_spec.rb

@@ -95,7 +95,7 @@ module Pod
           diff "${PODS_ROOT}/../Podfile.lock" "${PODS_ROOT}/Manifest.lock" > /dev/null
           if [[ $? != 0 ]] ; then
               cat << EOM
-          error: The sandbox is not in sync with the Podfile.lock. Run 'pod install'.
+          error: The sandbox is not in sync with the Podfile.lock. Run 'pod install' or update your CocoaPods installation.
           EOM
               exit 1
           fi