(maint) re-work the travis test matrix
(maint) re-work the travis test matrix

This commit re-works the Travis test matrix. Specifically:
  - remove support for Ruby 1.8.7
  - remove support for Puppet 2.x
  - bump Ruby 2.1.0 to Ruby 2.1.5

dmitryilyin/puppetlabs-chronos,dmitryilyin/puppetlabs-chronos,puppetlabs/puppetlabs-chronos,dmitryilyin/puppetlabs-chronos