ci: bump MSRV

Required to support the #[non_exhaustive] attribute.

Author: tmplt

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
 
         include:
           # Test MSRV
-          - rust: 1.38.0
+          - rust: 1.40.0
 
           # Test nightly but don't fail
           - rust: nightly