Stud is only in >=precise.

Change-Id: Ieb302e80af69a783736f2ebbdc9077e2cafe6a35
diff --git a/files/apts/tls-proxy b/files/apts/tls-proxy
index dce9c07..0a44015 100644
--- a/files/apts/tls-proxy
+++ b/files/apts/tls-proxy
@@ -1 +1 @@
-stud
+stud # only available in dist:precise,quantal