Update 32bit macOS again to xcode 9.3

os version 10.13 "High Sierra" appears to be the oldest release now for which Homebrew provides a gcc package.
Anything older and the Travis job will run out of time building gcc from source

Author: martin-frbg

.travis.yml

@@ -169,7 +169,7 @@ matrix:
         - BTYPE="BINARY=64 INTERFACE64=1"
 
     - <<: *test-macos
-      osx_image: xcode9.2
+      osx_image: xcode9.3
       env:
         - BTYPE="BINARY=32"