Update to 2018 edition

.github/workflows/ci.yml

@@ -72,28 +72,22 @@ jobs:
         if: matrix.os != 'windows'
 
   build:
-    name: Rust 1.34.0
+    name: Rust ${{matrix.rust}}
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        rust: [1.31.0, 1.34.0]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v3
-      - uses: dtolnay/rust-toolchain@1.34.0
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{matrix.rust}}
       - run: cd serde && cargo build --features rc
       - run: cd serde && cargo build --no-default-features
       - run: cd serde && cargo build
 
-  more:
-    name: Rust 1.30.0
-    runs-on: ubuntu-latest
-    timeout-minutes: 45
-    steps:
-      - uses: actions/checkout@v3
-      - uses: dtolnay/rust-toolchain@1.30.0
-        # Work around failing to parse manifest because editions are unstable.
-      - run: sed -i /test_suite/d Cargo.toml
-      - run: cd serde && cargo build --no-default-features
-      - run: cd serde && cargo build
-
   derive:
     name: Rust 1.56.0
     runs-on: ubuntu-latest