If you suddenly get a lot of cryptic errors in travis like `Errno::EINVAL: Errno::EINVAL: Invalid argument - bind(2) for "fe80::4001:aff:::fff" port 9515` then maybe this will save you 5 hours

monkeypatch_selenium.rb

# Monkeypatch Selenium Webdriver to work on Travis IPv6 infrastructure
module Selenium
  module WebDriver
    class PortProber
      IGNORED_ERRORS = [Errno::EADDRNOTAVAIL, Errno::EINVAL, Errno::EACCES].freeze
      IGNORED_ERRORS << Errno::EBADF if Platform.cygwin?

      def self.free?(port)
        interfaces = Socket.getaddrinfo('localhost', 8080).map { |e| e[3] }
        interfaces.each do |host|
          begin
            TCPServer.new(host, port).close
          rescue *IGNORED_ERRORS => ex
            $stderr.puts "port prober could not bind to #{host}:#{port} (#{ex.message})" if $DEBUG
            # ignored - some machines appear unable to bind to some of their interfaces
          end
        end

        true
      rescue
        false
      end
    end # PortProber
  end # WebDriver
end # Selenium