Corrected minimum Rust version to 1.54 in README.

Author: alexliesenfeld

README.md

@@ -9,7 +9,7 @@
 [![codecov](https://codecov.io/gh/alexliesenfeld/httpmock/branch/master/graph/badge.svg)](https://codecov.io/gh/alexliesenfeld/httpmock)
 [![crates.io](https://img.shields.io/crates/d/httpmock.svg)](https://crates.io/crates/httpmock)
 [![Mentioned in Awesome](https://camo.githubusercontent.com/e5d3197f63169393ee5695f496402136b412d5e3b1d77dc5aa80805fdd5e7edb/68747470733a2f2f617765736f6d652e72652f6d656e74696f6e65642d62616467652e737667)](https://github.com/rust-unofficial/awesome-rust#testing)
-[![Rust](https://img.shields.io/badge/rust-1.47%2B-blue.svg?maxAge=3600)](https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1540-2021-07-29)
+[![Rust](https://img.shields.io/badge/rust-1.54%2B-blue.svg?maxAge=3600)](https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1540-2021-07-29)
 
 </div>