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