README.md: Correct the Minimum Supported Rust Version to 1.48

Author: Techcable

README.md

@@ -20,8 +20,9 @@
   <a href="https://docs.rs/releases/search?query=slog-">
       <img src="https://docs.rs/slog/badge.svg" alt="docs-rs: release versions documentation">
   </a>
-  <a href="https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html">
-      <img src="https://img.shields.io/badge/rust-1.31%2B-orange.svg">
+  <!-- Badge showing our Minimum Supported Rust Version, along with a link to the release announcement on the official blog -->
+  <a href="https://blog.rust-lang.org/2020/12/31/Rust-1.49.0.html">
+      <img src="https://img.shields.io/badge/rust-1.49%2B-orange.svg">
   </a>
   <br>
     <strong><a href="https://github.com/slog-rs/slog/wiki/Getting-started">Getting started</a></strong>