Bump MSRV to 1.48.0 like rust-bitcoin did

Author: stevenroose

.github/workflows/rust.yml

@@ -11,7 +11,7 @@ jobs:
                 include:
                     - rust: stable
                     - rust: nightly
-                    - rust: 1.41.1
+                    - rust: 1.48.0
         steps:
             - name: Checkout Crate
               uses: actions/checkout@v2

README.md

@@ -47,4 +47,4 @@ The following versions are officially supported and automatically tested:
 * 0.21.0
 
 # Minimum Supported Rust Version (MSRV)
-This library should always compile with any combination of features on **Rust 1.41.1**.
+This library should always compile with any combination of features on **Rust 1.48.0**.