travis: Use newer xcode

.travis.yml

@@ -7,10 +7,7 @@ matrix:
     - os: linux
       compiler: clang
     - os: osx
-      osx_image: xcode7.3
-      compiler: clang
-    - os: osx
-      osx_image: xcode7
+      osx_image: xcode8.3
       compiler: clang
 sudo: false
 addons: