(PUP-3272) Make failing Ruby 1.8.7 tests conditional

With the assymetric serialization/deserialization in facts.rb that now
require psych support that is unavailable on Ruby 1.8.7 (using syck)
there are two tests that fail on Ruby 1.8.7.

These failures are real - Puppet 4.0.0 will not support Ruby 1.8.7. As a
temporary solution these tests were made conditional on Ruby 1.8.7.