appveyor: update ruby versions

appveyor.yml

@@ -4,8 +4,12 @@ skip_tags: true
 
 environment:
   matrix:
-    - ruby_version: "23"
-    - ruby_version: "23-x64"
+    - ruby_version: "24"
+    - ruby_version: "24-x64"
+    - ruby_version: "25"
+    - ruby_version: "25-x64"
+    - ruby_version: "26"
+    - ruby_version: "26-x64"
 
 install:
   - SET PATH=C:\Ruby%ruby_version%\bin;%PATH%
@@ -18,7 +22,4 @@ install:
 test_script:
   - rake
 
-artifacts:
-  - path: pkg\*.gem
-
 build: off