Remove superfluous code from serverspec example

serverspec does not use `RSpec.configuration.os` anymore, see https://github.com/serverspec/serverspec/commit/797b14015d5a881f5fea531e2df44db672fc1e08#commitcomment-4760496

Also, `pathname` is required, but not used anywhere.
Merge pull request #3 from mlafeldt/patch-1

Remove superfluous code from serverspec example