Updates to .travis.yml, Gemfile, Rakefile and CONTRIBUTING.md

This commit does the following:
 - Reverts removing Puppet 2.7.0 from the travis testing matrix
 - Adds the ability for the Gemfile template to parse groups and makes
   the template more human readable
 - Breaks the Gemfile configs into unit test and system test groups and
   updates travis to not install system test gems
 - Added 'relative' and removed 'disable_clas_parameter_defaults' from
   linter checks
 - Updates CONTRIBUTING.md with correct URLS
Add json gem to default gems

The puppet_facts gem has a non-explicit dependency on the json gem. On
Ruby 1.8.7 with Puppet 2.7, json is not included by default and there
is no hiera gem to explicitly include it, so we'll add it to our
Gemfile.
Update config_defaults.yml