zproject_travis.gsl : default distro should match the one specified in matrix tests to minimize the surprises

Author: jimklimov

zproject_travis.gsl

@@ -33,8 +33,12 @@ cache:
 os:
 - linux
 
-#dist:
-#- xenial
+# The default distro for env:/matrix: tests and unspecified matrix: tests;
+# for the latter see also addons/apt/sources with the package repos in use.
+# See https://docs.travis-ci.com/user/reference/overview/ for up-to-date
+# allowed values of "dist" supported by the Travis CI service.
+dist:
+- xenial
 
 #services:
 #- docker