Update .travis.yml, Gemfile, Rakefile, and CONTRIBUTING.md
Add json gem

The puppet_facts gem implicitly depends on the json gem.
On Ruby 1.8.7, json is not built in. On Puppet 2.7, it is
not explicitly pulled in by the hiera gem. So we add it
here explicitly.
Merge pull request #391 from cmurphy/master

Update .travis.yml, Gemfile, Rakefile, and CONTRIBUTING.md
Purge apt::pin when apt::source is to be purged
Correctly pass priority to apt::pin