Revert "Workaround for broke CI with YARD 0.9.20"

This reverts commit 36d736b.

Author: koic

Gemfile

@@ -12,7 +12,4 @@ gem 'rspec'
 gem 'rubocop', github: 'rubocop-hq/rubocop'
 gem 'rubocop-performance', '~> 1.5.0'
 gem 'rubocop-rspec', '~> 1.29.0'
-# Workaround for YARD 0.9.20 or lower.
-# It specifies `github` until the release that includes the following changes:
-# https://github.com/lsegal/yard/pull/1290
-gem 'yard', github: 'lsegal/yard', ref: '10a2e5b'
+gem 'yard', '~> 0.9'