[ruby/irb] Reidline mode needs Reline what needs String#grapheme_clusters

https://github.com/ruby/irb/commit/2b0b19b87c

lib/irb/input-method.rb

@@ -293,5 +293,5 @@ module IRB
     def encoding
       @stdin.external_encoding
     end
-  end
+  end if RUBY_VERSION >= "2.5.0" # Reline needs String#grapheme_clusters
 end