Travis: Use XCode 9.3 / macOS 10.13 for osx/iphone builds

.travis.yml

@@ -33,9 +33,11 @@ matrix:
       compiler: gcc
     - env: GODOT_TARGET=osx TOOLS=yes CACHE_NAME=${GODOT_TARGET}-clang-tools
       os: osx
+      osx_image: xcode9.3
       compiler: clang
     - env: GODOT_TARGET=iphone TOOLS=no CACHE_NAME=${GODOT_TARGET}-clang
       os: osx
+      osx_image: xcode9.3
       compiler: clang
 
 addons: