Run CI build/tests with 1.40.0

This change makes CI run with rust 1.40.0 as minimum supported Rust
version for the crate.

Signed-off-by: Matthias Beyer <[email protected]>

Author: matthiasbeyer

.github/workflows/msrv.yml

@@ -9,6 +9,7 @@ jobs:
     strategy:
       matrix:
         rust:
+          - 1.40.0
           - stable
           - beta
           - nightly
@@ -35,6 +36,7 @@ jobs:
     strategy:
       matrix:
         rust:
+          - 1.40.0
           - stable
           - beta
           - nightly