Build issue possibly related to a go-plugin-helpers update

[pramukta]

Seems like this repo may just be an example, so not sure if it matters much, but anyway: I think an interface was updated or something in the go-plugin-helpers repo. This is preventing the plugin for building. I tried on OS X as well as Ubuntu 16.04. The failure that is reported first is:

nfsDriver does not implement volume.Driver (missing Capabilities method)

I have a patch that gets it building, but haven't written any go before, so hopefully someone else can take a look? Will have a PR posted shortly.