Use Xcode 10.3 for TensorFlow

It's also compatible and a bit newer, so should work better with macOS Catalina.

FYI @meteorcloudy
diff --git a/buildkite/pipelines/tensorflow-postsubmit.yml b/buildkite/pipelines/tensorflow-postsubmit.yml
index dc25467..9f0e47c 100644
--- a/buildkite/pipelines/tensorflow-postsubmit.yml
+++ b/buildkite/pipelines/tensorflow-postsubmit.yml
@@ -24,7 +24,7 @@
     environment:
       TF_IGNORE_MAX_BAZEL_VERSION: 1
       USE_BAZEL_VERSION: latest
-    xcode_version: "10.2.1"
+    xcode_version: "10.3"
     shell_commands:
     # - |-
     #   echo '