Update CI config (#26)

Now that the macOS 11 runner is gone, there's simply no way to test Rust
versions older than 1.54 in GitHub Actions any more.

Also run tests in release mode to ensure that optimizations don't break
any logic or compilation.

Author: sagebind

.github/workflows/ci.yml

@@ -8,4 +8,5 @@ jobs:
   test:
     uses: sagebind/workflows/.github/workflows/rust-ci.yml@v1
     with:
-      msrv: "1.38"
+      msrv: "1.54"
+      test-release: true