Move back to XCode 9.2

.travis.yml

@@ -28,11 +28,11 @@ matrix:
           env: SPEC=android-g++ CONFIG=installer
           sudo: false
         - os: osx
-          osx_image: xcode10.1
+          osx_image: xcode9.2
           env: SPEC=macx-clang CONFIG=installer
           sudo: required
         - os: osx
-          osx_image: xcode10.1
+          osx_image: xcode9.2
           env: SPEC=macx-ios-clang CONFIG=release
           sudo: false
 # OSX builds pared back to installer only since travis sucks so bad we can't afford more than one'