Define travis matrix in .sync.yml

Since the other modules need to wait for a major release before
deprecating Puppet 2.7 support, this module's travis.yml is out of sync
and will need to stay that way for a while.
Merge pull request #390 from cmurphy/puppet_27

Define travis matrix in .sync.yml
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