@google-cloud/vision does not support proxy with HTTP(S)_PROXY


#### Environment details

  - OS: Docker
  - Node.js version: 7.10.0
  - npm version: 4.2.0
  - google-cloud-node version: google-cloud/vision: ^0.11.2

#### Steps to reproduce

* set HTTP(S)_PROXY via ENV
* block all outgoing traffic w/o proxy try to detect or annotate via nodejs api

expected: response from google server
actual: no response/block by firewall
