travis: Update to Debian Stretch

Update to Stretch to use updated version of glib2.0. Fixes CI
failure when parsing error output in tests/test-config.sh. See:

https://github.com/ostreedev/ostree/pull/1838#issuecomment-482186680
https://api.travis-ci.org/v3/job/518830164/log.txt

Closes: #1839
Approved by: cgwalters

ci/travis-install.sh

@@ -48,7 +48,7 @@ NULL=
 # Typical values for ci_distro=ubuntu: xenial, trusty
 # Typical values for ci_distro=debian: sid, jessie
 # Typical values for ci_distro=fedora might be 25, rawhide
-: "${ci_suite:=jessie}"
+: "${ci_suite:=stretch}"
 
 # ci_configopts: Additional arguments for configure
 : "${ci_configopts:=}"