use proper version of malicious plugin from rubygems

Gemfile

@@ -1,5 +1,3 @@
 source "https://rubygems.org"
 
 gemspec
-
-gem "jekyll_test_plugin_malicious", :github => "jekyll/jekyll-test-plugin-malicious"

github-pages.gemspec

@@ -35,5 +35,5 @@ msg
   s.add_development_dependency("rspec", "~> 3.3")
   s.add_development_dependency("rubocop", "~> 0.35")
   s.add_development_dependency("pry", "~> 0.10")
-  s.add_development_dependency("jekyll_test_plugin_malicious", "~> 2.0")
+  s.add_development_dependency("jekyll_test_plugin_malicious", "~> 0.2")
 end