changes: file for 31919's bionic Travis image change

changes/ticket31919_bionic

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+  o Minor features (continuous integration):
+    - Use Ubuntu Bionic images for our Travis CI builds, so we can get
+      a recent version of coccinelle. But leave chutney on Ubuntu Trusty,
+      until we can fix some Bionic permissions issues (see ticket 32240).
+      Related to ticket 31919.