Plugin is not compatible because latest version of `public_suffix` does not support ruby 2.0

[majormoses]

This was reported in #31 I think we need to do several things based on this:

• for 1.x versions of the plugin we should backport pinning to ~> 2.0.5
• for 2.x versions of the plugin we should pin to ~> 2.0.5 and make a release
• after we have made a release that provides backwards compatibility we should update the list of ruby versions we support as ruby 2.0, 2.1, and 2.2 are all EOL from ruby and per our policy our goal is to provide support for supported versions of ruby.
• release a major version of sensu-plugin that drops ruby 2.0, 2.1, and 2.2 support and pull that in