chor: update xcode in travis tests to 9.3

Author: lini

.travis.yml

@@ -21,8 +21,8 @@ matrix:
     - os: osx
       env: 
         - BuildiOS="11"
-        - Xcode="9.2"
-      osx_image: xcode9.2
+        - Xcode="9.3"
+      osx_image: xcode9.3
       language: node_js 
       node_js: "8"
       jdk: oraclejdk8
@@ -49,7 +49,7 @@ matrix:
       language: node_js
       node_js: "8"
       jdk: oraclejdk8
-      osx_image: xcode8.3
+      osx_image: xcode9.3
       before_script:
         - cd seed-tests && npm i
       script: travis_wait travis_retry npm run test.ios