Exclude some more "development only" files from the gem package

especially, js files under assets dir are very huge, but used only for
precompiling assets, and not referenced in production.

simplecov-html.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |gem|
   gem.files = Dir.chdir(__dir__) do
     `git ls-files -z`.split("\x0").reject do |f|
       (File.expand_path(f) == __FILE__) ||
-        f.start_with?(*%w[bin/ test/ spec/ features/ .git .github appveyor Gemfile])
+        f.start_with?(*%w[bin/ test/ spec/ features/ .git .github appveyor Gemfile assets .rubocop.yml Guardfile])
     end
   end
   gem.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")