Use the preserve_order feature of rust-yaml

This improves the output from the yaml writer because the fields are not sorted alphabetically. e.g. 'type' is not at the bottom.
servo · bors-servo · Feb 13, 2017 · Feb 11, 2017 · Feb 11, 2017 · 86b8452ba73bd5d7e86a172bddc1157d855ca8d6
commit 86b8452ba73bd5d7e86a172bddc1157d855ca8d6
diff --git a/Cargo.lock b/Cargo.lock
diff --git a/wrench/Cargo.toml b/wrench/Cargo.toml
@@ -16,7 +16,7 @@ app_units = "0.3"
 image = "0.12"
 clap = { version = "2", features = ["yaml"] }
 lazy_static = "0.2"
-yaml-rust = { git = "https://github.com/vvuk/yaml-rust" }
+yaml-rust = { git = "https://github.com/vvuk/yaml-rust", features = ["preserve_order"] }
 serde_json = "0.8"
 time = "0.1"
 crossbeam = "0.2"