Revert "Bump claimed MSRV to 1.39"

Apparently the version actually required by the time/unstable_kv
feature is much higher than 1.39. In fact, by my tests it appears to
start working around 1.53

This makes it worthwhile to test it seperately and restore the old MSRV.

This reverts commit 17d94eb.

Cargo.toml

@@ -21,7 +21,7 @@ edition = "2018"
 #
 # The first version of Cargo that supports this field was in Rust 1.56.0.
 # In older releases, the field will be ignored, and Cargo will display a warning.
-rust-version = "1.39"
+rust-version = "1.38"
 
 [lib]
 path = "lib.rs"

README.md

@@ -11,8 +11,8 @@
       <img src="https://img.shields.io/gitter/room/slog-rs/slog.svg" alt="slog-rs Gitter Chat">
   </a>
 
-  <a href="https://blog.rust-lang.org/2019/11/07/Rust-1.39.0.html">
-      <img src="https://img.shields.io/badge/rust-1.39%2B-orange.svg" alt="Minimum Rust Version 1.38">
+  <a href="https://blog.rust-lang.org/2019/09/26/Rust-1.38.0.html">
+      <img src="https://img.shields.io/badge/rust-1.38%2B-orange.svg" alt="Minimum Rust Version 1.38">
   </a>
 </p>