update MSRV badge

Author: Hexilee

README.md

@@ -8,7 +8,7 @@
 [![Rust Docs](https://docs.rs/async-postgres/badge.svg)](https://docs.rs/async-postgres)
 [![Crate version](https://img.shields.io/crates/v/async-postgres.svg)](https://crates.io/crates/async-postgres)
 [![Download](https://img.shields.io/crates/d/async-postgres.svg)](https://crates.io/crates/async-postgres)
-[![Version](https://img.shields.io/badge/rustc-1.39+-lightgray.svg)](https://blog.rust-lang.org/2019/11/07/Rust-1.39.0.html)
+[![MSRV-1.40](https://img.shields.io/badge/MSRV-1.40-blue.svg)](https://blog.rust-lang.org/2019/12/19/Rust-1.40.0.html)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://github.com/Hexilee/async-postgres/blob/master/LICENSE)
 
   </p>