(maint) Pins redcarpet to < 3.0.0 for Ruby 1.8.7

The redcarpet gem 3.0.0 version does not work in Ruby 1.8.7.

https://github.com/vmg/redcarpet/issues/280
(#22233) Catch json load error for gce when no json gem present

When the specs were run in an environment without the json gem, the
gce_spec.rb would fail, because it was now throwing a LoadError.

Splits the spec into two, guarded by Facter.json? for the expected cases
with and without json present.
Merge pull request #525 from jpartlow/issue/master/22233-fix-json-in-gce-spec

Issue/master/22233 fix json in gce spec