Final v2.1.0 release

* Remove useless `if defined` surrounding `include xinetd` in `server.pp`
* Final _CHANGELOG_ modifications
* require rspec < 3.0.0 for test suite as rspec-puppet seems to have no support for 3.x version
* Fix issue when two remctl::server::commands were declared; one with ensure absent, the other one with ensure present. If the absent one was handled first, the `concat` object would have `ensure => absent`. This would conflict with the `ensure => present` other.
Add travis test for puppet 3.6