Raise minimum tested version to 1.40

1.39 is still supported but tracing-attributes in our test deps assumes 1.40.

    error[E0658]: use of unstable library feature 'option_flattening'
       --> tracing-attributes-0.1.10/src/lib.rs:933:10
        |
    933 |         .flatten();
        |          ^^^^^^^
        |
        = note: for more information, see rust-lang/rust#60258

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [beta, stable, 1.39.0]
+        rust: [beta, stable, 1.40.0]
         include:
           - rust: nightly
             rustflags: --cfg async_trait_nightly_testing
@@ -25,6 +25,14 @@ jobs:
         env:
           RUSTFLAGS: ${{matrix.rustflags}}
 
+  msrv:
+    name: Rust 1.39.0
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@1.39.0
+      - run: cargo check
+
   clippy:
     name: Clippy
     runs-on: ubuntu-latest