build-rina: also test on latest Debian release

Now that the test suite passes again for Jessie, let's try Stretch.

software/build-rina/software.cfg

@@ -72,7 +72,7 @@ sparse-checkout = /playbook/roles/rina
 [vm-debian]
 # building a generic Debian kernel uses a lot of space
 size = 16Gi
-dists = debian-jessie
+dists += debian-jessie
 packages +=
 # generic (another SR that build packages automatically would use the same list)
   apt-utils build-essential devscripts equivs lsb-release