Raise preserve_order required Rust version to 1.32

Required by the dependency on indexmap which now depends on hashbrown 0.8.1.

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [beta, stable, 1.31.0]
+        rust: [beta, stable, 1.32.0, 1.31.0]
         os: [ubuntu]
         include:
           - rust: stable
@@ -42,6 +42,7 @@ jobs:
           toolchain: ${{matrix.rust}}
       - run: cargo check
       - run: cargo check --features preserve_order
+        if: matrix.rust != '1.31.0'
       - run: cargo check --features float_roundtrip
       - run: cargo check --features arbitrary_precision
       - run: cargo check --features raw_value