Do not interpolate attributes in default.rb

Given the recipe tries to overwrite default attributes: 
```
node.default['maven']['3']['version'] = '3.1.1'
include_recipe 'maven::default'
```
When chef-client evaluates `attributes/default.rb` for the first time:
`default['maven']['3']['url'] = "http://apache.mirrors.tds.net/maven/maven-3/#{node['maven']['3']['version']}/binaries/apache-maven-#{node['maven']['3']['version']}-bin.tar.gz"`

the attributes are not overwritten yet, it happens later it the process: http://docs.opscode.com/essentials_cookbook_attribute_files.html#Attribute Precedence

it still thinks that `node['maven']['3']['version'] = 3.1.0` thus url is incorrectly interpolated as: 
'http://apache.mirrors.tds.net/maven/maven-3/3.1.0/binaries/apache-maven-3.1.0-bin.tar.gz'

The fix is to move the url interpolation to recipe.