Use Xcode 9.4 on Travis CI

See <https://blog.travis-ci.com/2018-07-19-xcode9-4-default-announce>.

.travis.yml

@@ -63,12 +63,10 @@ matrix:
 
     - env: GODOT_TARGET=osx TOOLS=yes CACHE_NAME=${GODOT_TARGET}-tools-clang
       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
 
     - env: GODOT_TARGET=server TOOLS=yes CACHE_NAME=${GODOT_TARGET}-tools-gcc