Ruby 2.5+ doesn't support option -rubygems anymore.

--- a/Rakefile
+++ b/Rakefile
@@ -18,7 +18,6 @@
 require 'rake/testtask'
 Rake::TestTask.new 'test' do |t|
   t.test_files = FileList['test/test_*.rb']
-  t.ruby_opts = ['-rubygems']
 end
 
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