[#39] Included the default attributes in `elasticsearch::proxy` to prevent Chef errors

In some environments, the Chef run failed with:

    ================================================================================
    Recipe Compile Error in /tmp/vagrant-chef-1/chef-solo-2/cookbooks/elasticsearch/attributes/proxy.rb
    ================================================================================

    NoMethodError
    -------------
    undefined method `[]' for nil:NilClass

    Cookbook Trace:
    ---------------
      /tmp/vagrant-chef-1/chef-solo-2/cookbooks/elasticsearch/attributes/proxy.rb:18:in `from_file'
    ...

as reported by @ncolomer in #39. There's a related [`CHEF-2903`](http://tickets.opscode.com/browse/CHEF-2903) issue
in the Chef tracking system (“Attribute files not loaded in deterministic order”).

This patch include the default attributes in the `elasticsearch::proxy` attribute file, as suggested by @chrisroberts.

Fixes #39