msrv: bump to Rust 1.52.1

This matches the latest stable release of Rust.

Author: BurntSushi

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
         include:
         - build: pinned
           os: ubuntu-18.04
-          rust: 1.41.0
+          rust: 1.52.1
         - build: stable
           os: ubuntu-18.04
           rust: stable