appveyor.yml: show the OS version

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64300 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
2018-08-11 14:15:58 +00:00 · 2018-08-11 14:15:58 +00:00 · 591b2566d7
--- a/appveyor.yml
+++ b/appveyor.yml
@ -8,6 +8,7 @@ environment:
  matrix:
    - vs: "120"
 install:
+  - ver
  - chcp
  - SET BITS=%Platform:x86=32%
  - SET BITS=%BITS:x=%