This PR fixes the following build error when using Psych 4.0.
```console
% ruby -v
ruby 3.1.0dev (2021-05-17T10:51:51Z master ee611341c9) [x86_64-darwin19]
% cd path/to/i18n
% bundle exec rake
(snip)
Finished in 1.310478s, 1265.1872 runs/s, 2004.6121 assertions/s.
1) Error:
KeyValueCacheFileTest#test_load_translations_caches_file_through_updated_modification_time:
I18n::InvalidLocaleData: can not load translations from
/var/folders/6j/5l8q3y250b97529_tcssrwlm0000gn/T/test20210528-81604-ijnu7v.yml:
#<Psych::DisallowedClass: Tried to load unspecified class: Symbol>
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:245:in
`rescue in load_yml'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:241:in
`load_yml'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:226:in
`load_file'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/cache_file.rb:23:in
`load_file'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:18:in
`block in load_translations'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:18:in
`each'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:18:in
`load_translations'
/Users/koic/src/github.com/ruby-i18n/i18n/test/backend/cache_file_test.rb:49:in
`block (2 levels) in <module:CacheFileTest>'
/Users/koic/.rbenv/versions/3.1.0-dev/lib/ruby/3.1.0/tempfile.rb:317:in
`open'
/Users/koic/src/github.com/ruby-i18n/i18n/test/backend/cache_file_test.rb:43:in
`block in <module:CacheFileTest>'
2) Error:
SimpleCacheFileTest#test_load_translations_caches_file_through_updated_modification_time:
I18n::InvalidLocaleData: can not load translations from
/var/folders/6j/5l8q3y250b97529_tcssrwlm0000gn/T/test20210528-81604-9kcvkm.yml:
#<Psych::DisallowedClass: Tried to load unspecified class: Symbol>
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:245:in
`rescue in load_yml'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:241:in
`load_yml'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:226:in
`load_file'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/cache_file.rb:23:in
`load_file'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:18:in
`block in load_translations'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:18:in
`each'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:18:in
`load_translations'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/simple.rb:79:in
`init_translations'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/simple.rb:89:in
`lookup'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n/backend/base.rb:32:in
`translate'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n.rb:207:in `block
in translate'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n.rb:203:in `catch'
/Users/koic/src/github.com/ruby-i18n/i18n/lib/i18n.rb:203:in
`translate'
/Users/koic/src/github.com/ruby-i18n/i18n/test/backend/cache_file_test.rb:50:in
`block (2 levels) in <module:CacheFileTest>'
/Users/koic/.rbenv/versions/3.1.0-dev/lib/ruby/3.1.0/tempfile.rb:317:in
`open'
/Users/koic/src/github.com/ruby-i18n/i18n/test/backend/cache_file_test.rb:43:in
`block in <module:CacheFileTest>'
1658 runs, 2627 assertions, 0 failures, 2 errors, 0 skips
rake aborted!
```
This breaking is because an incompatibility with Psych 4.0.
- ruby/psych#487
- https://bugs.ruby-lang.org/issues/17866
For example, this PR solves the problem of faker gem that depend on i18n gem.
faker-ruby/faker#2330