title: Capistrano 3 "upload" task raises error
body: <p>I have capistrano 3 task</p>
  
  <pre><code>namespace :db do
    desc "serurely manages dbconn file after deploy"
    task :dbconfig do
      on roles(:web) do |host|
        upload("config/deploy/dbconn.template.php", "#{shared_path}/dbconn.php")
      end
    end
  end
  </code></pre>
  
  <p>When running 
  <code>cap staging db:dbconfig</code>
  I get error</p>
  
  <pre><code>   cap aborted!
     undefined method `upload' for #&lt;SSHKit::Backend::Netssh:0x007ffe9a84b9f8&gt;
  </code></pre>
  
  <p>According to capistrano API I can use this method to upload files, i also found it in source code of sshkit lib</p>
  
  <p>gem list  </p>
  
  <pre><code>*** LOCAL GEMS ***
  bundler (1.3.5)
  capistrano (3.0.1)
  CFPropertyList (2.2.0)
  i18n (0.6.5)
  libxml-ruby (2.6.0)
  net-scp (1.1.2)
  net-ssh (2.7.0)
  nokogiri (1.5.6)
  rake (10.1.0)
  sqlite3 (1.3.7)
  sshkit (1.1.0)
  term-ansicolor (1.2.2)
  tins (0.13.1)
  </code></pre>