Chefspec 4.1 deprecation warnings

The recently released chefspec 4.1 deprecates syntax that is used in this cookbook's spec and in some of its indirect dependencies (see opscode-cookbooks/windows#132). As a consequence running specs with chefspec 4.1 results in hundred of lines of deprecation warnings.

This PR does:
 - fix 'should' deprecation warnings by converting them to rspec 3.0 'expect' syntax
 - lock chef-spec version to < 4.1 until the deprecation is fixed in this cookbook's dependencies.

Once all dependencies are updated, I will add a final changeset to this PR to make the specs fully compatible with chefspec 4.1.