[ruby/psych] Prepare to develop 5.0.0

https://github.com/ruby/psych/commit/c3b5183f42

ext/psych/lib/psych/versions.rb

@@ -2,7 +2,7 @@
 
 module Psych
   # The version of Psych you are using
-  VERSION = '4.0.4.dev'
+  VERSION = '5.0.0.dev'
 
   if RUBY_ENGINE == 'jruby'
     DEFAULT_SNAKEYAML_VERSION = '1.28'.freeze