update travis' xcode to v9

Author: hawkrives

.travis.yml

@@ -63,7 +63,7 @@ matrix:
 
     - os: osx
       language: objective-c
-      osx_image: xcode8.3
+      osx_image: xcode9
       node_js: '8'
       env: [IOS=yes, CAN_DEPLOY=yes]
 
@@ -104,7 +104,7 @@ before_install:
       nvm install "$TRAVIS_NODE_VERSION";
       nvm use "$TRAVIS_NODE_VERSION";
     fi
-  
+
   - echo "node -v is now $(node -v)"
 
   # turn off fancy npm stuff