Enforce UTF-8 locale when running tests in appveyor

chcp 65001 mean Code Page Number and 65001 is UTF-8.

So by doing this change we modify the code page to UTF-8 for the
commande that follows.

See documentation: https://ss64.com/nt/chcp.html
See related issue: appveyor/ci#2107

Author: benoittgt

appveyor.yml

@@ -29,7 +29,7 @@ before_test:
   - bundle --version
 
 test_script:
-  - bundle exec rspec --backtrace
+  - chcp 65001 && bundle exec rspec --backtrace
 
 environment:
   matrix: