Make explicit the reference to standard Hash

the old version defines a new class called Hash inside the hashie module in the following manner: "class Hash < Hash". This is fine, and the second Hash there refers to ruby's Hash, but if for some reason the class is being reloaded it produces a superclass mismatch error. Defining the class like this: "class Hash < ::Hash" disambiguates Hash and allows this code to be reloaded multiple times.

Thanks to @meh who made a similar commit to master - but it's needed in the stable branch (and a new release is needed as well)