Stud is available in new distributions

Stud was marked as precise-only. In reality it's only not available in
lucid. All supported distributions are ok now.

partial blueprint devstack-https
Change-Id: Ie86f345102cfddcca5387be514b67641ef750313
diff --git a/files/apts/tls-proxy b/files/apts/tls-proxy
index 8fca42d..dce9c07 100644
--- a/files/apts/tls-proxy
+++ b/files/apts/tls-proxy
@@ -1 +1 @@
-stud # only available in dist:precise
+stud