Travis: update osx images

.travis.yml

@@ -103,11 +103,19 @@ matrix:
     - compiler: gcc
       env: SCRIPT=coverage
     - os: osx
-      osx_image: xcode6.4
+      osx_image: xcode7.3
       compiler: clang
       env: SCRIPT=cmake
     - os: osx
-      osx_image: xcode7.3
+      osx_image: xcode8.3
+      compiler: clang
+      env: SCRIPT=cmake
+    - os: osx
+      osx_image: xcode9.4
+      compiler: clang
+      env: SCRIPT=cmake
+    - os: osx
+      osx_image: xcode10
       compiler: clang
       env: SCRIPT=cmake SANITIZE=address
     - env: SCRIPT=arduino VERSION=1.6.7 BOARD=arduino:avr:uno