Try to add workaround for warnings

```
.../ext/psych/lib/psych/versions.rb:4: warning: already initialized constant Psych::VERSION
.../.ext/common/psych/versions.rb:4: warning: previous definition of VERSION was here
```

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64599 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/psych/lib/psych/versions.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 module Psych
   # The version is Psych you're using
-  VERSION = '3.1.0.pre1'
+  VERSION ||= '3.1.0.pre1'
 
   if RUBY_ENGINE == 'jruby'
     DEFAULT_SNAKEYAML_VERSION = '1.21'.freeze