Update Travis config and set 1.24 as minimal supported rustc version

.travis.yml

@@ -1,8 +1,19 @@
 language: rust
-rust:
-  - stable
-  - beta
-  - nightly
 matrix:
+  include:
+    # Minimal supported rustc version
+    - rust: 1.24.0
+      script:
+        # Build only the library (examples may fail)
+        - cargo build
+    - rust: stable
+    - rust: beta
+    - rust: nightly
+    - rust: nightly-2018-03-07
+      script:
+        # nostd build
+        - cargo build --no-default-features --features core_io
+        - cargo build --no-default-features --features core_io,alloc,core_io/collections
+
   allow_failures:
     - rust: nightly

README.md

@@ -5,6 +5,7 @@ Rust FAT FS
 [![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE.txt)
 [![crates.io](http://meritbadge.herokuapp.com/fatfs)](https://crates.io/crates/fatfs)
 [![Documentation](https://docs.rs/fatfs/badge.svg)](https://docs.rs/fatfs)
+[![Minimum rustc version](https://img.shields.io/badge/rustc-1.24+-yellow.svg)](https://blog.rust-lang.org/2018/02/15/Rust-1.24.html)
 
 FAT filesystem library implemented in Rust.