Update rake test:unit to run on ruby2.7

`ubygems.rb' is deprecated, and will be removed on or after 2018-12-01. Remove `-rubygems' from your command-line, or use `-r rubygems' instead

Author: fliiiix

Rakefile

@@ -53,7 +53,7 @@ task :man => 'man/rdiscount.1'
 require 'rake/testtask'
 Rake::TestTask.new('test:unit') do |t|
  t.test_files = FileList['test/*_test.rb']
- t.ruby_opts += ['-rubygems'] if defined? Gem
+ t.ruby_opts += ['-r rubygems'] if defined? Gem
 end
 task 'test:unit' => [:build]