.circleci: Bump xcode workers to 9.4.1 (#74)

Summary:
Signed-off-by: Eli Uriegas <[email protected]>

Pull Request resolved: #74

Reviewed By: pbelevich

Differential Revision: D23428404

Pulled By: seemethere

fbshipit-source-id: b2d45b06519349fa8e2521b579929f6ad4f4e62e

Author: seemethere

.circleci/config.yml

@@ -157,7 +157,7 @@ jobs:
   binary_macos_wheel:
     <<: *binary_common
     macos:
-      xcode: "9.0"
+      xcode: "9.4.1"
     steps:
       - checkout_merge
       - run:
@@ -179,7 +179,7 @@ jobs:
   binary_macos_conda:
     <<: *binary_common
     macos:
-      xcode: "9.0"
+      xcode: "9.4.1"
     steps:
       - checkout_merge
       - run: