allow setting of virtualenv version

Closes #74

Author: coderanger

attributes/default.rb

@@ -42,3 +42,4 @@
 default['python']['pip_location'] = "#{node['python']['prefix_dir']}/bin/pip"
 default['python']['virtualenv_location'] = "#{node['python']['prefix_dir']}/bin/virtualenv"
 default['python']['setuptools_version'] = nil # defaults to latest
+default['python']['virtualenv_version'] = nil

recipes/virtualenv.rb

@@ -22,4 +22,5 @@
 
 python_pip "virtualenv" do
   action :upgrade
+  version node['python']['virtualenv_version']
 end