capybara_ssl_server.rb

require 'webrick/https'
require 'rack/handler/webrick'

def run_ssl_server(app, port)

  opts = {
    :Port => port,
    :SSLEnable => true,
    :SSLVerifyClient => OpenSSL::SSL::VERIFY_NONE,
    :SSLPrivateKey => OpenSSL::PKey::RSA.new("./spec/support/server.key"),
    :SSLCertificate => OpenSSL::X509::Certificate.new(File.read "./spec/support/server.crt"),
    :SSLCertName => [["US", 'localhost']],
    :AccessLog => [], 
    :Logger => WEBrick::Log::new(Rails.root.join("./log/capybara_test.log").to_s)
  }

  Rack::Handler::WEBrick.run(app, opts)
end

Capybara.server do |app, port|
  run_ssl_server(app, port)
end

There is an error on line #10. You need to include File.read to read the contents of the file just like in the line after that.