Update preserve_order required compiler to 1.38.0 for hashbrown

Required by indexmap introducing a dependency on hashbrown 0.11, which
does not build with rustc 1.36.0.

    error[E0658]: use of unstable library feature 'ptr_cast'
       --> git.luolix.top-1ecc6299db9ec823/hashbrown-0.11.2/src/raw/mod.rs:504:57
        |
    504 |         NonNull::new_unchecked(self.table.ctrl.as_ptr().cast())
        |                                                         ^^^^

    error[E0658]: use of unstable library feature 'ptr_cast'
        --> git.luolix.top-1ecc6299db9ec823/hashbrown-0.11.2/src/raw/mod.rs:1275:51
         |
    1275 |         NonNull::new_unchecked(self.ctrl.as_ptr().cast())
         |                                                   ^^^^

    error[E0658]: use of unstable library feature 'ptr_cast'
      --> git.luolix.top-1ecc6299db9ec823/hashbrown-0.11.2/src/raw/sse2.rs:49:40
       |
    49 |         Group(x86::_mm_loadu_si128(ptr.cast()))
       |                                        ^^^^

    error[E0658]: use of unstable library feature 'ptr_cast'
      --> git.luolix.top-1ecc6299db9ec823/hashbrown-0.11.2/src/raw/sse2.rs:59:39
       |
    59 |         Group(x86::_mm_load_si128(ptr.cast()))
       |                                       ^^^^

    error[E0658]: use of unstable library feature 'ptr_cast'
      --> git.luolix.top-1ecc6299db9ec823/hashbrown-0.11.2/src/raw/sse2.rs:69:34
       |
    69 |         x86::_mm_store_si128(ptr.cast(), self.0);
       |                                  ^^^^

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [beta, stable, 1.45.0, 1.40.0, 1.36.0, 1.31.0]
+        rust: [beta, stable, 1.45.0, 1.40.0, 1.38.0, 1.31.0]
         os: [ubuntu]
         include:
           - rust: stable
@@ -43,6 +43,8 @@ jobs:
       - run: cargo check
       - run: cargo check --features preserve_order
         if: matrix.rust != '1.31.0'
+      - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,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
@@ -64,7 +66,6 @@ jobs:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@1.36.0
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc
-      - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,preserve_order
 
   clippy:
     name: Clippy