[VAGRANT] Fix incorrect Chef installation for CentOS

The script was also doing `yum update`, not really neccessary :), copy&paste fail
[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