(#13439) Fix MRI 1.9 issue with spec_helper

(#13439) Fix MRI 1.9 issue with spec_helper

When using MRI 1.9.x the stdlib spec helper does not invoke because
Puppet.settings.private_methods returns symbols instead of strings.

This is a problem because we need to set default configuration settings
like Puppet[:vardir] when using the compiler.

This patch fixes the issue by simply checking the Puppet version.  This
seems a better choice than rescuing NoMethodError since the method might
be renamed or removed in the future.