Raise minimum compiler to 1.38

Required by indexmap's dependency on hashbrown:

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

.github/workflows/ci.yml

@@ -22,11 +22,11 @@ jobs:
       - run: cargo test
 
   msrv:
-    name: Rust 1.31.0
+    name: Rust 1.38.0
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: dtolnay/rust-toolchain@1.31.0
+      - uses: dtolnay/rust-toolchain@1.38.0
       - run: cargo check
 
   clippy: