Upgrade to rustc 1.20.0-nightly (504328a31 2017-07-21)

and cargo 0.21.0-nightly (ffab51954 2017-07-18).

The latter adds support for the `[patch]` section of `Cargo.toml`:

http://doc.crates.io/manifest.html#the-patch-section
http://doc.crates.io/specifying-dependencies.html#overriding-dependencies

Cargo.toml

@@ -14,8 +14,8 @@ opt-level = 3
 # debug = true
 # lto = false
 
-[replace]
+[patch.crates-io]
 # If you need to temporarily test Servo with a local fork of some upstream
 # crate, add that here. Use the form:
 #
-#     "<crate>:<version>" = { path = "/path/to/local/checkout" }
+#     <crate> = { path = "/path/to/local/checkout" }

rust-commit-hash

@@ -1 +1 @@
-ab91c70cc69450bc69b76df0fd2faa82bda42d6b
+504328a31a211814f9cac6de84c5a7ed154f58eb