_only_ update the image (#925)

* _only_ update the image

* Update .travis.yml

Co-authored-by: Paul Kehrer <paul.l.kehrer@gmail.com>
diff --git a/.travis.yml b/.travis.yml
index 48a74cc..4b3f05f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,7 +14,7 @@
   include:
   - language: generic
     os: osx
-    osx_image: xcode11.3
+    osx_image: xcode11.6
     env: TOXENV=py27
   - python: "2.7"
     env: TOXENV=py27