Update appveyor Ruby

To work around:

```
Gem::InstallError: nokogiri requires Ruby version < 2.5, >= 2.2.

An error occurred while installing nokogiri (1.8.0), and Bundler cannot continue.
Make sure that `gem install nokogiri -v '1.8.0'` succeeds before bundling.
```

appveyor.yml

@@ -5,8 +5,8 @@ skip_tags: true
 environment:
   JRUBY_OPTS: "--dev -J-Xmx1024M --debug"
   matrix:
-    - ruby_version: "Ruby21"
-    - ruby_version: "Ruby21-x64"
+    - ruby_version: "Ruby24"
+    - ruby_version: "Ruby24-x64"
 
 cache:
   - vendor/bundle