(maint) only install libjson-ruby for ruby 1.8.7

JSON is included in stdlib beginning with Ruby 1.9.2. Therefore,
for our testing, it isn't needed except on platforms whose system
Ruby is 1.8.7.

Acceptance results for this PR are available at:
https://jenkins.puppetlabs.com/view/Hiera/view/Master/job/Hiera-Acceptance-master/116/

After review and merge, please merge up into master.