Update to a more modern macOS image (#909)

diff --git a/.travis.yml b/.travis.yml
index 5cc9418..5734206 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,7 +14,7 @@
   include:
   - language: generic
     os: osx
-    osx_image: xcode10.1
+    osx_image: xcode11.3
     env: TOXENV=py27
   - python: "2.7"
     env: TOXENV=py27