Travis: Use Ubuntu Bionic, but keep Trusty for chutney

Bionic has a recent coccinelle version, which passes our CI tests.

But Bionic (and Xenial) cause permissions errors for chutney.
We'll fix those in 32240.

Part of 31919.

.travis.yml

@@ -34,6 +34,10 @@ matrix:
       os: osx
     ## We run chutney on Linux, because it's faster than chutney on macOS
     - env: CHUTNEY="yes" CHUTNEY_ALLOW_FAILURES="2" SKIP_MAKE_CHECK="yes"
+      ## (Linux only) Use an older Linux image (Ubuntu Trusty)
+      ## The Xenial and Bionic images cause permissions issues for chutney,
+      ## this is a workaround, until we fix #32240.
+      dist: trusty
     ## We check asciidoc with distcheck, to make sure we remove doc products
     ## We use Linux clang, because there are no other Linux clang jobs
     - env: DISTCHECK="yes" ASCIIDOC_OPTIONS="" SKIP_MAKE_CHECK="yes"
@@ -58,8 +62,8 @@ matrix:
     - env: CHUTNEY="yes" CHUTNEY_ALLOW_FAILURES="2" SKIP_MAKE_CHECK="yes"
       os: osx
 
-## (Linux only) Use the latest Linux image (Ubuntu Trusty)
-dist: trusty
+## (Linux only) Use a recent Linux image (Ubuntu Bionic)
+dist: bionic
 
 ## Download our dependencies
 addons: