Feature: Add net-ssh to list of gems reported by cap doctor

[mattbrictson]

Right now the doctor:gems task reports gem versions for the following gems:

• capistrano
• airbrussh
• rake
• sshkit
• all gems starting with capistrano-

I think we should add net-ssh to this list, because it is a crucial part of Capistrano's behavior, and certain versions of net-ssh have been known to cause bugs (e.g. capistrano/sshkit#357).

If someone wants to try doing a PR for this, here are some pointers:

• Add net-ssh here:

  capistrano/lib/capistrano/doctor/gems_doctor.rb

  Line 37 in b5e4aa6

  %w(capistrano airbrussh rake sshkit) & Gem.loaded_specs.keys

• Add a corresponding test, like this:

  capistrano/spec/lib/capistrano/doctor/gems_doctor_spec.rb

  Lines 25 to 28 in b5e4aa6

  	it "prints the SSHKit version" do
  	expect { doc.call }.to\
  	output(/sshkit\s+#{Regexp.quote(SSHKit::VERSION)}/).to_stdout
  	end

• Update the CHANGELOG