Logstash is not able to find the flapjack plugin which has been installed in vendor\local_gems

Hi All, I have output plugin for flapjack in path vendor\local_gems. Even if when i am trying to list all the gems, i am not able to see the local flapjack gem. After installation of plugin, the below entries are there in Gemfile.
gem "logstash-output-flapjack", "0.1.5", :path => "vendor/local_gems/0472ffe3/logstash-output-flapjack-0.1.5"

But the folder vendor/local_gems/0472ffe3/logstash-output-flapjack-0.1.5 is empty. I am not getting any gem file inside the above package. I have used the below commands to install the output plugin.

gem build logstash-output-flapjack.gemspec
bin/plugin install logstash-output-flapjack-0.1.5.gem

So when I am trying to start the logstash by using below command
bin/logstash -f logstash-filter.conf

I am getting the message that its not able to find the output block of flapjack. It means logstash is not able to locate the plugin for flapjack.

Please help.
Thanks In Advance.

Now its working, Made some changes in logstash-output-flapjack.gemspec.
Added the below lines:
s.files
= Dir[ 'lib/logstash/outputs/.rb', 'spec/**/.rb']

Thanks
Sailesh