Set connection.keep_alive_timeout using idle_timeout in ruby 2.0.

Fixes issue #39

## Problem

Ruby 2.0 added keep_alive_timeout, which behaves the same way as idle_timeout, except that the default timeout is 5 seconds rather than 2 seconds.  This means that the default timeout is effectively 2 seconds, and idle_timeout isn't respected when it is greater than 2 seconds.

## Solution

Set keep_alive_timeout on the connection using idle_timeout on ruby 2.0.