simplify spec_helper_acceptance.rb

Currently, modules required to be installed are installed without any version specified into spec_helper_acceptance.rb.

It is better to specify version, accordingly with metadata.json, like :

install_module_from_forge('puppetlabs-stdlib', '>= 4.0.0 < 5.0.0') 
install_module_from_forge('puppetlabs-apt', '>= 5.0.0 < 6.0.0') if fact('os.family') == 'Debian'