(GEP-179) Handle dropped support for node inheritance

Node inheritance is no longer supported in Puppet versions >= 4.0.
This commit ensures that Geppetto will warn about this deprecation
in older versions (can be ignored by changing default preference
setting). Versions >= 4.0 will always report errors.
Merge pull request #42 from puppetlabs/issue-179

(GEP-179) Handle dropped support for node inheritance