Travis: bump XCode version to 9.2

Author: Morwenn

.travis.yml

@@ -114,7 +114,7 @@ matrix:
       compiler: clang
 
     - os: osx
-      osx_image: xcode8.3
+      osx_image: xcode9.2
       env: BUILD_TYPE=Release CMAKE_GENERATOR="Xcode"
       addons:
         homebrew:

README.md

@@ -79,7 +79,7 @@ gfx::timsort(collection, std::less<std::string>{}, &len);
 The library has been tested with the following compilers:
 * GCC 5
 * Clang 3.8
-* The AppleClang version that ships with Xcode 8.3
+* Xcode 9.2 AppleClang
 * MSVC 2017 update 9
 
 It should also work with more recent compilers, and most likely with some older compilers too.