add apt-transport-https for contrib/docker/Dockerfile for GPU on Ubuntu 16.04 (#953)
add apt-transport-https for contrib/docker/Dockerfile for GPU on Ubuntu 16.04 (#953)

* workaround to avoid apt-get update error for GPU docker image build on Ubuntu 16.04: The method driver /usr/lib/apt/methods/https could not be found.

* removed extraneous proxy variables from the contrib/docker/Dockerfile for GPU on Ubuntu 16.04

NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph