message: Update metabuild test to 2018.
diff: @@ -715,7 +715,7 @@ fn metabuild_json_artifact() {
  "crate_types": [
  "bin"
  ],
  - "edition": "2015",
  + "edition": "2018",
  "kind": [
  "custom-build"
  ],
  @@ -762,7 +762,7 @@ fn metabuild_failed_build_json() {
  "crate_types": [
  "bin"
  ],
  - "edition": "2015",
  + "edition": "2018",
  "kind": [
  "custom-build"
  ],
  