Allow unstable suite for mendel packages

Change-Id: I057f6c9002e2cf75cce90a01007c766d062c2f9f
diff --git a/lintian/vendors/mendel/main/data/changes-file/known-dists b/lintian/vendors/mendel/main/data/changes-file/known-dists
index 4a5ab8b..3d6fa72 100644
--- a/lintian/vendors/mendel/main/data/changes-file/known-dists
+++ b/lintian/vendors/mendel/main/data/changes-file/known-dists
@@ -3,3 +3,4 @@
 mendel-animal
 mendel-beaker
 mendel-chef
+unstable