Roll back travis host to focal

.travis.yml

@@ -10,7 +10,7 @@
 # virt: lxd             # use a lxd container
 # arch: amd64           # optional, this is the default, routes to a full VM
 # os: linux             # optional, this is the default
-dist: jammy # xenial (Ubuntu 16) is the default; focal is Ubuntu 20; jammy is Ubuntu 22
+dist: focal # xenial (Ubuntu 16) is the default; focal is Ubuntu 20; jammy is Ubuntu 22
 
 vm:
   size: x-large

travis/travis_libdap_build_offset.sh

@@ -15,7 +15,7 @@ loggy "BEGIN $HR"
 
 # This is the TravisCI build number when the
 # last formal release was built.
-export LIBDAP_TRAVIS_BUILD_OFFSET=1914
+export LIBDAP_TRAVIS_BUILD_OFFSET=1917
 
 if [ "$TRAVIS_PULL_REQUEST" != "false" ]
 then