Prepare for inclusion into nanoc org

This PR fixes a few small issues and makes the repo ready to be included in the nanoc organisation on GitHub.

* Removed mentions of the plugin template
* Fixed dependencies so they depend on the current version of nanoc, rather than nanoc 4.0.
* Fixed issue with arrays/strings in executing the command.
* Removed the base64 test, as it fails on my local machine, and I think a single test should be enough.