(PUP-6545) Add 2.3 to Appveyor test matrix

Previously only ruby 2.1.8 was used in the the Appveyor tests.  This commit adds
Ruby 2.3.0 to the test matrix.  The complete and up to date list of installed
software can be found at;
https://www.appveyor.com/docs/installed-software

appveyor.yml

@@ -1,16 +1,22 @@
 version: 4.1.0.{build}
 clone_depth: 10
-init:
-  - SET
 install:
-  - SET PATH=C:\Ruby21-x64\bin;%PATH%
+build: off
+
+environment:
+  matrix:
+  - RUBY_VER: 23-x64
+  - RUBY_VER: 21-x64
+
+test_script:
+  - SET
+  - SET PATH=C:\Ruby%RUBY_VER%\bin;%PATH%
   - SET LOG_SPEC_ORDER=true
   - bundle install --jobs 4 --retry 2 --without development extra
   - type Gemfile.lock
   - ruby -v
-build: off
-test_script:
   - bundle exec rspec spec
+
 on_failure:
   - appveyor PushArtifact .\spec_order.txt
 notifications: