Improvements to plugin manager

- Use better installer method to avoid documentation installation
  With the previous method it would install the documentation which didn't exist.

- Create Fake gemspec info for logstash to help dependency management for plugins
  Previously we would create a real gem and install it to aid in dependency management for plugins and logstash
  Now we create a fake gemspec on the fly with the real version of logstash avoiding having to build it every time.