Set STDOUT.sync=true when testing on AppVeyor [skip travis]

This issue is a correction error at 5a682bfc.

appveyor.yml

@@ -40,5 +40,5 @@ init:
 
 build_script:
   - set MRUBY_CONFIG=appveyor_config.rb
-  - rake -E "$stdout.sync=true" -m -j4
-  - rake test
+  - rake -m -j4
+  - rake -E STDOUT.sync=true test