Workaround for broke CI with YARD 0.9.20 when using Ruby 2.7.0-dev

This PR solves the following "The manual directory is out of sync" error
when using Ruby 2.7.0-dev.

```console
% ruby -v
ruby 2.7.0dev (2019-12-03T05:51:14Z master e42d9d8df8) [x86_64-darwin17]
% CI=true bundle exec rake

(snip)

The manual directory is out of sync. Run `rake
generate_cops_documentation` and commit the results.
```

https://circleci.com/gh/rubocop-hq/rubocop-minitest/968

This issue will be resolved with the release of YARD 0.9.21 or higher,
which includes the following changes:
lsegal/yard#1290

Author: koic

Gemfile

@@ -10,4 +10,7 @@ gem 'bump', require: false
 gem 'rake'
 gem 'rubocop', github: 'rubocop-hq/rubocop'
 gem 'rubocop-performance', '~> 1.5.0'
-gem 'yard', '~> 0.9'
+# 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'