Deep dream example on plaidml

Hi all,

I'm trying the deep dream example and I have it running on tensorflow/CPU backend.

I'm on Debian, I'm having a bad time with CUDA, I'm generally happier with plaidml running on OpenCL.

If I change the start of the code to:

 library(keras)
library(tensorflow)
use_backend("plaidml")
 library(purrr)

Then I get the following error at the following lines:

> model <- application_incepti .... [TRUNCATED] 
INFO:plaidml:Opening device "geforce_gtx_960.0"
Error in py_call_impl(callable, dots$args, dots$keywords) : 
  KeyError: 'plaidml'