ci(travis): update osx xcode image (#55)

diff --git a/.travis.yml b/.travis.yml
index 7bdfba4..e490a4c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -23,7 +23,7 @@
 
 _ios: &_ios
   os: osx
-  osx_image: xcode10.3
+  osx_image: xcode11.6
 
 _android: &_android
   language: android
@@ -117,4 +117,3 @@
     else
       $PARAMEDIC_COMMAND --config ./pr/$PLATFORM --plugin $PARAMEDIC_PLUGIN_TO_TEST --buildName $PARAMEDIC_BUILDNAME;
     fi
-