(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.
Merge pull request #220 from rji/maint/stable/exclude-json-ruby193

(maint) only install libjson-ruby for ruby 1.8.7