Test with Rust 1.40.0 instead of Rust 1.39.0

The build failures seen in [1] are because of use of `cfg(doctest)` in
what appears to be a transitive dependency - `remove_dir_all`, via
`rusty-fork`, via `proptest`.

`cfg(doctest)` was made stable in Rust 1.40, so this commit bumps the
version of Rust against which we test to match that to ensure this is
the only failure.

[1]: https://github.com/danburkert/prost/pull/326/checks?check_run_id=786898150

.github/workflows/continuous-integration-workflow.yaml

@@ -46,7 +46,7 @@ jobs:
       matrix:
         toolchain:
           - stable
-          - 1.39.0
+          - 1.40.0
         os:
           - ubuntu-latest
           - macos-latest