CI: Bump minimum supported Rust version to 1.23

A change in protobuf made this necessary.

.appveyor.yml

@@ -16,7 +16,7 @@ environment:
       target: x86_64-pc-windows-msvc
     - channel: stable
       target: i686-pc-windows-msvc
-    - channel: 1.18.0
+    - channel: 1.23.0
       target: x86_64-pc-windows-msvc
 
 install:

.travis.yml

@@ -6,7 +6,7 @@ rust:
   - stable
   - beta
   - nightly
-  - 1.18.0
+  - 1.23.0
 script:
   - cargo test --verbose
   - cargo test --verbose --no-default-features