I'm having trouble with exec input plugin. The command I'm trying to execute is 'papertrail -json' which is a ruby gem I have installed locally, so when I run the command by itself in terminal I do get the desired json output. When I run logstash it throws this error: `block in verify_gemfile_dependencies_are_found!': You have requested (Bundler::GemNotFound) logstash-core = 2.3.2'. Any suggestions on how to overcome this?