Use Xcode 10.1 for BuildKite

PiperOrigin-RevId: 243266760
diff --git a/.bazelci/presubmit.yml b/.bazelci/presubmit.yml
index e0946ea..3cf36df 100644
--- a/.bazelci/presubmit.yml
+++ b/.bazelci/presubmit.yml
@@ -1,6 +1,7 @@
 ---
 platforms:
   macos:
+    xcode_version: "10.1"
     build_targets:
     - "//:tulsi"
     test_flags: