Fix readme merge example by using Hash#update

Squashes the following:

commit 05453998b0dd7f42c52d9d997f9e2827c574ccbf
Author: Ismael Celis <ismaelct@gmail.com>
Date:   Thu May 1 16:18:05 2014 +0100

    Hash#update metter than #merge!

commit d9608137913effcde5f1cbf29ad4499c2c6497dc
Author: Ismael Celis <ismaelct@gmail.com>
Date:   Thu May 1 16:15:33 2014 +0100

    Fix readme merge example