Once again disable unstable test.

rubygem-activesupport.spec

@@ -77,6 +77,10 @@ for f in test/evented_file_update_checker_test.rb; do
   mv $f{,.disable}
 done
 
+# This seems to be unstable as well ...
+# https://github.com/rails/rails/issues/25682
+sed -i '/def test_iso8601_output_and_reparsing$/,/^  end$/ s/^/#/' test/core_ext/duration_test.rb
+
 memcached &
 mPID=$!
 ruby -Ilib:test -e 'Dir.glob "./test/**/*_test.rb", &method(:require)'
@@ -98,6 +102,7 @@ popd
 %changelog
 * Tue Aug 22 2017 Vít Ondruch <vondruch@redhat.com> - 1:5.1.3-2
 - Explicitly require rubygem(json).
+- Once again disable unstable test.
 
 * Tue Aug 08 2017 Pavel Valena <pvalena@redhat.com> - 1:5.1.3-1
 - Update to Active Support 5.1.3.