Update to Ruby 2.6.0

[olliebennett]

The official Ruby docker image has released version 2.6.0.

Any chance of re-building this image using the latest ruby?

Yep, I know that (as described in #34) we could fork this ourselves, but would appreciate an official update! :)

[radeno]

You can use these images https://cloud.docker.com/repository/docker/smikino/ruby-node
As a contributor into Ruby-Node when @Starefossen merge #32 and rebuild it.