CI: Bump minimum supported Rust version to 1.24.1

because of lazy_static.

.appveyor.yml

     - channel: stable
       target: i686-pc-windows-msvc
       cargoflags: --no-default-features
-    - channel: 1.23.0
+    - channel: 1.24.1
       target: x86_64-pc-windows-msvc
 
 install:

.travis.yml

   - stable
   - beta
   - nightly
-  - 1.23.0
+  - 1.24.1
 script:
   - |
-      if [ "$TRAVIS_RUST_VERSION" == "1.23.0" ]; then
+      if [ "$TRAVIS_RUST_VERSION" == "1.24.1" ]; then
         cargo test --verbose;
         cargo doc --verbose --no-deps;
       else