Bump the minimum required Psych version

The existing code already breaks with older versions and this just
reflects the minimum version that works in the gemspec.

Psych v4.0.1 is the one introducing `YAML.safe_dump` which we're using
to store LogEntry serialized details.

Fixes #5320.

core/solidus_core.gemspec

@@ -42,7 +42,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'mini_magick', '~> 4.10'
   s.add_dependency 'monetize', '~> 1.8'
   s.add_dependency 'kt-paperclip', ['>= 6.3', '< 8']
-  s.add_dependency 'psych', ['>= 3.1.0', '< 5.0']
+  s.add_dependency 'psych', ['>= 4.0.1', '< 5.0']
   s.add_dependency 'ransack', '~> 2.0'
   s.add_dependency 'sprockets-rails'
   s.add_dependency 'state_machines-activerecord', '~> 0.6'