Use xcode11 for travis CI.
diff --git a/.travis.yml b/.travis.yml
index e709618..58eedce 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -34,7 +34,7 @@
             - extra-android-m2repository 
       # static check
       - env: TEST_SUITE=static_code_analysis OCLINT=true
-        osx_image: xcode7.2
+        osx_image: xcode11.2
         language: objective-c
       - env: TEST_SUITE=static_code_analysis ANDROID_LINT=true
         language: android