travis: force usage of Xcode 8.3 image

Travis has upgraded the default Xcode images from 8.3 to 9.4 on 31st
July 2018, including an upgrade to macOS 10.13. Unfortunately, this
breaks our CI builds on our maintenance branches. As we do not want to
include mayor changes to fix the integration right now, we force use of
the old Xcode 8.3 images.

.travis.yml

@@ -37,6 +37,7 @@ addons:
     - valgrind
 
 sudo: false
+osx_image: xcode8.3
 
 matrix:
  fast_finish: true