travis: Use Xcode 9.2

Uses the latest available Xcode version. This allows the use of more
C++17 facilities without the CI failing.

.travis.yml

@@ -24,7 +24,7 @@ matrix:
     - os: osx
       env: NAME="macos build"
       sudo: false
-      osx_image: xcode7.3
+      osx_image: xcode9.2
       install: "./.travis/macos/deps.sh"
       script: "./.travis/macos/build.sh"
       after_success: "./.travis/macos/upload.sh"