Add unmaintained crate advisory for block-cipher-trait (RUSTSEC-2020-0018) (#310)

* Add unmaintained crate advisory for `block-cipher-trait`

It's been renamed to `block-cipher`. See:

RustCrypto/traits#139

* Assign RUSTSEC-2020-0018 to block-cipher-trait

Author: tarcieri

crates/block-cipher-trait/RUSTSEC-2020-0018.toml

@@ -0,0 +1,18 @@
+[advisory]
+id = "RUSTSEC-2020-0018"
+package = "block-cipher-trait"
+title = "crate has been renamed to `block-cipher`"
+informational = "unmaintained"
+date = "2020-05-26"
+url = "https://github.com/RustCrypto/traits/pull/139"
+description = """
+This crate has been renamed from `block-cipher-trait` to `block-cipher`.
+
+The new repository location is at:
+
+<https://github.com/RustCrypto/traits/tree/master/block-cipher>
+"""
+
+[versions]
+patched = []
+unaffected = []