Update travis xcode version

Author: Martin Schwamberger

.travis.yml

@@ -5,7 +5,7 @@ matrix:
       services:
         - docker
     - os: osx
-      osx_image: xcode10
+      osx_image: xcode10.2
 
 env: 
   - RELEASE_FILES=$TRAVIS_BUILD_DIR/target/*.zip