[#39] Fixed the `print_value` extension method to be more resilient

Use the `[]` accessor method, not the `send` method,
to fight `Chef::Node::Attribute` quirks, such as:

    [10] pry(#<Chef::Recipe>)> a.timeout
    => "10s"

    [11] pry(#<Chef::Recipe>)> a.send('timeout')
    ArgumentError: wrong number of arguments (0 for 1)
    from (pry):11:in `timeout'

    [13] pry(#<Chef::Recipe>)> a.method(:timeout).source_location
    => ["/opt/vagrant_ruby/lib/ruby/1.8/timeout.rb", 100]

Added `node.elasticsearch.discovery.zen.ping.timeout` attribute to Vagrantfile
and a test case to `installation_test.rb`.

Fixes #39.