[CLIENT] Added Manticore transport for JRuby platforms

Manticore [https://github.com/cheald/manticore] is a JRuby HTTP client
built on the Apache HttpClient 4.x components.

This patch adds it as a transport implementation, so you can use it as a HTTP client on JRuby:

    require 'elasticsearch/transport/transport/http/manticore'

    client = Elasticsearch::Client.new transport_class: Elasticsearch::Transport::Transport::HTTP::Manticore;

    client.transport.connections.first.connection
    => #<Manticore::Client:0x56bf7ca6 ...>

    client.info['status']
    => 200

Closes #107