Update to MXNet 1.6

[fhieber]

This is still work in progress as MXNet 1.6 is not yet available.

• Removed old CUDA support, MXNet 1.6 requires CUDA 9.1, cudnn 7.5.
• Fixed numpy version.

Pull Request Checklist

• Changes are complete (if posting work-in-progress code, prefix your pull request title with '[WIP]'
  until you can check this box.
• Unit tests pass (pytest)
• Were system tests modified? If so did you run these at least 5 times to account for the variation across runs?
• System tests pass (pytest test/system)
• Passed code style checking (./style-check.sh)
• You have considered writing a test
• Updated major/minor version in sockeye/__init__.py. Major version bump if this is a backwards incompatible change.
• Updated CHANGELOG.md

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[fhieber]

This seems ready to merge.