gnucash-docs master: Tell travis to use ubuntu-18.04-docker file

[Geert Janssens]

Updated	 via  https://github.com/Gnucash/gnucash-docs/commit/533a3c80 (commit)
	from  https://github.com/Gnucash/gnucash-docs/commit/cfef19d7 (commit)



commit 533a3c805f88c7b4eced59a24dbe24ae34c63dba
Author: Geert Janssens <geert at kobaltwit.be>
Date:   Mon Jun 8 19:24:18 2020 +0200

    Tell travis to use ubuntu-18.04-docker file



Summary of changes:
 .travis.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)