upgraded xcode image

.travis.yml

@@ -49,27 +49,27 @@ matrix:
         - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
 
     - os: osx
-      osx_image: xcode8
+      osx_image: xcode9
       env:
         - MATRIX_EVAL="brew install cmake && CC=gcc-4.9 && CXX=g++-4.9"
 
     # - os: osx
-    #   osx_image: xcode8
+    #   osx_image: xcode9
     #   env:
     #     - MATRIX_EVAL="brew install gcc5 && CC=gcc-5 && CXX=g++-5"
 
     # - os: osx
-    #   osx_image: xcode8
+    #   osx_image: xcode9
     #   env:
     #     - MATRIX_EVAL="brew install gcc6 && CC=gcc-6 && CXX=g++-6"
 
     - os: osx
-      osx_image: xcode8
+      osx_image: xcode9
       env:
         - MATRIX_EVAL="brew install gcc cmake && CC=gcc-7 && CXX=g++-7"
 
     - os: osx
-      osx_image: xcode8
+      osx_image: xcode9
       env:
         - MATRIX_EVAL="brew install cmake && CC=clang && CXX=clang"