Indication in readme for which MXNet versions are supported

[tjad]

I haven't been able to see other than by dates of commits which version of MXNet this gem requires. Could indication be made.

Reason for needing to know is for the c api compatability. As far as I'm aware there have been significant internal changes to MXNet, including the deprecation of the Symbolic api

[tjad]

I see in the CI Rakefile some indication that the version this project was last bound against is 1.2.1

mxnet_version = ENV['mxnet_version'] || '1.2.1.post1'

[tjad]

Docker file suggest 1.3.1

mxnet.rb/docker/Dockerfile.erb

Line 4 in edc85f6

ARG MXNET_VERSION=1.3.1