[ruby/optparse] Prefer `require_relative`

https://github.com/ruby/optparse/commit/bb08cd47a8

lib/optparse.rb

@@ -1115,7 +1115,7 @@ XXX
     Switch::OptionalArgument.new do |pkg|
       if pkg
         begin
-          require 'optparse/version'
+          require_relative 'optparse/version'
         rescue LoadError
         else
           show_version(*pkg.split(/,/)) or