Use mrkn/yard temporarily

The latest yard doesn't support the decorator `module_function`.
I submitted lsegal/yard#1365 to add the support of it.
We need to use mrkn/yard until the pull-request will be merged and the
new version of yard will be released.

Author: mrkn

.github/workflows/ci.yml

@@ -52,8 +52,8 @@ jobs:
 
     - run: bundle install --jobs 4 --retry 3
 
-    - run: rake
+    - run: bundle exec rake
 
-    - run: rake build
+    - run: bundle exec rake build
 
     - run: gem install pkg/*.gem

Gemfile

@@ -1,3 +1,6 @@
 source "https://rubygems.org/"
 
 gemspec
+
+# Temporary use this for module_function decorator support
+gem "yard", github: "mrkn/yard", branch: "module_function_decorator"