Question: I was reading about Go, and I can see that it's very good and can be a language used by many developers in some months, but I want to decide a simple thing: **Learn Go or improve my Python or Ruby knowledge?**

Years developing with Python: 1  

Years developing with Ruby: 0.3

Answer: 
It's up to you. You should probably do both if you can, because that way you will have more tools on your metaphorical programmer's belt.

There are a number of things that I think are worth considering whenever I'm in a similar dilemma.

Is a new language (not just unfamiliar, but actually new, like Go) likely to catch on? (If so, it will become practically NECESSARY to learn it, rather than just a good idea.)

You will need to spend some time to learn the unfamiliar language. Will this time investment result in some sort of positive return? The obvious one here is development time (i.e., can you eventually get more done and get back the time you spent to learn it), but if the language is superior in other ways (runs faster or with less memory, is best for your particular problem domain) those might factor in too.

Will learning the unfamiliar language allow you to solve a relevant/important/urgent problem that cannot be solved with what you already know?

Unfortunately, none of us can tell you how to weigh each of these concerns. You'll need to think about it really carefully and come to the answer on your own.
IMO you should improve your Python knowledge. Python is a widely adopted language, whereas
Go is still at a very, very early stage and there's no reason to believe that it will become successful.

From a purely utilitarian perspective, you will get a lot more value from learning Python.