[CHORE] Upgrade Mac CI image to 10.2.1 (#914)

.circleci/config.yml

@@ -44,7 +44,7 @@ jobs:
 
   e2e-test:
     macos:
-      xcode: "10.1.0"
+      xcode: "10.2.1"
 
     environment:
       BASH_ENV: "~/.nvm/nvm.sh"
@@ -172,7 +172,7 @@ jobs:
 
   ios-build:
     macos:
-      xcode: "10.1.0"
+      xcode: "10.2.1"
 
     environment:
       BASH_ENV: "~/.nvm/nvm.sh"
@@ -240,7 +240,7 @@ jobs:
 
   ios-testflight:
     macos:
-      xcode: "10.1.0"
+      xcode: "10.2.1"
 
     steps:
       - checkout