Prepare release to support agent 5.0

* Adds puppetlabs/ruby dependency, to ensure rubygems installation
properly for reporting with the dogapi gem
* Remove compatibility with CentOS 5.x (missing ruby gems)
* Change repos to support new 5.0 agent
* Improves error message when dogapi gem is missing
* Use HTTPS transport in the agent
* Remove python dependencies because 5.0 is self-contained
* Ensure that the agent is installed when using integrations
* Use unambiguous full access path to variables as recommended by Puppet
* Whitespace nitpicks