Whitelist languages/powershell which allows cookbooks that use dsc_scri...

This whitelists 'powershell' under 'languages'.

Without information about powershell, ChefSpec testing of anything using the new dsc_script resource in Chef 11.16.0+ fails because the provider insists on there being ohai data about Powershell v4.0:

https://github.com/opscode/chef/blob/11.16.4/lib/chef/platform/query_helpers.rb#L49-52