Docker version in README #565

Greetings.

I am not even sure this is worth an issue, but the second paragraph of the README mentions that
this gem is supposed to work with Docker 1.4.*.

When I run docker --version, I get 19.03.13, so this seems like a reason not to use this gem.

Yet, by looking at the repo as a whole, it looks active, and git blame tells me that README line
is from 2 years ago.

Am I missing something, i.e., did I look at the wrong version number?
What is the Docker currently supported?

Thanks in advance,
Nathan