Make the plugin Chef 11 compatible

* Removed unused require (that's lucky, since Chef 11 doesn't have that class anymore)
* Allow any version between 10.14 that's smaller than 12

I'm not sure about pinning the version to < 12, maybe it would be safer to set it to the latest 11 version. But then you have to create a new version for every new chef release...

What do you think?