gnucash master: Change ubuntu to 18.04 in .travis.yml.

[John Ralls]

Updated	 via  https://github.com/Gnucash/gnucash/commit/3a6ea8a5 (commit)
	from  https://github.com/Gnucash/gnucash/commit/d55d0a0e (commit)



commit 3a6ea8a59f054615510f4e7bcf8ca4fb9b855aba
Author: John Ralls <jralls at ceridwen.us>
Date:   Fri Sep 6 12:54:30 2019 -0700

    Change ubuntu to 18.04 in .travis.yml.



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