[FIX] Fix incorrect attribute setting in default.rb

The default.rb attributes file used Chef 11 incompatible syntax for setting the node default attribute,
throwing the `Chef::Exceptions::ImmutableAttributeModification` exception.

See: http://www.opscode.com/blog/2013/02/05/chef-11-in-depth-attributes-changes/

Related: #68, #69

attributes/default.rb

@@ -5,8 +5,8 @@
 
 # Initialize the node attributes with node attributes merged with data bag attributes
 #
-node.set[:elasticsearch] ||= {}
-node.set[:elasticsearch]   = DeepMerge.merge(node[:elasticsearch].to_hash, settings.to_hash)
+node.default[:elasticsearch] ||= {}
+node.default[:elasticsearch]   = DeepMerge.merge(node.default[:elasticsearch].to_hash, settings.to_hash)
 
 # === VERSION AND LOCATION
 #

recipes/aws.rb

@@ -1,3 +1,3 @@
-node.set[:elasticsearch][:plugin][:mandatory] = node[:elasticsearch][:plugin][:mandatory] | ['cloud-aws']
+node.default[:elasticsearch][:plugin][:mandatory] = node[:elasticsearch][:plugin][:mandatory] | ['cloud-aws']
 
 install_plugin "elasticsearch/elasticsearch-cloud-aws/#{node.elasticsearch[:plugins][:aws][:version]}"