Ask HN: What to learn now for a Ruby / Rails developer?

First of all - please humor me if the question is somewhat colloquially phrase - I approached this as a question I would ask a knowledgeable acquaintance at the coffee-shop.

I know and love Rails and have enjoyed using Ruby last few years. In particular I enjoyed the fact that Ruby has so many different libraries, tools ported / adapted to it - I could do NLP projects, ML projects even some OpenCV stuff. Rails was great for quickly deploying fun web projects even as a joke for friends and family or for testing out some APIs. It seems like RoR is at it's dusk and I need to move on though.

What would be a good framework / language to learn to catch up with state of the art web programming?

I know Javascript, Python so am looking for something new that has potential to become as universal as RoR has been (was? still is?)

I looked into Go and it's appealing but it seems like it has more of a 'build everything from scratch' which is great, but not so fun when I need to build something quickly to interact with some API.

Ideally I am looking for something that would complement my foray into machine learning which I am attempting right now. So.. is there a ML-friendly language that has somewhat 'universal' ambitions, has web frameworks and enthusiastic community?