(#11115) Support spec tests under rspec 2.7.x

Without this patch, the puppet spec tests do not run under rspec 2.7.x.

This patch fixes this issue by using a proper require expression to
include the spec_helper module in spec/unit/network/resolver_spec.rb

This patch changes a single line in spec/unit/network/resolver_spec.rb
as the other tests already require the spec_helper the *right* way.