Don't show untagged PRs

As far as I can tell, `add_pr_wo_labels` wasn't actually
implemented. Implementing this per the documentation.

Signed-off-by: Tom Duffield <tom@chef.io>

lib/github_changelog_generator/generator/generator.rb

@@ -81,7 +81,7 @@ module GitHubChangelogGenerator
         log += issues_to_log(issues, pull_requests)
       end
 
-      if options[:pulls]
+      if options[:pulls] && options[:add_pr_wo_labels]
         # Generate pull requests:
         log += generate_sub_section(pull_requests, options[:merge_prefix])
       end