fix minimum rust version

- *serde* configuration attributes require MRV >= 1.18.0
  - introduced in commit 67f173d; "Optional serde serialization feature for colours and styles"
- `cargo test` now includes transitive dependencies which require MRV >= 1.28.0
- use of the `#[must_use]` configuration attribute causes compiler warnings
  - warnings appear for compilers >= 1.22.0 and < 1.28.0
  - introduced in commit afe5c93; "Mark Style::paint and Colour::paint as #[must_use]"

.appveyor.yml

@@ -26,7 +26,7 @@ environment:
 
   matrix:
   # minimum version
-  - CHANNEL: 1.17.0
+  - CHANNEL: 1.28.0
     ARCH: i686
     ABI: msvc
   # "msvc" ABI