First, I don't think that Rails (and similar) will become obsolete in a few years. Personally, if it's a serious project, I would go with the stack you know the best.

Now, if you're willing to learn something new, I would recommend that you try Phoenix http://phoenixframework.org/. It similar to Rails in structure, and Elixir (built on top of Erlang VM) is also similar to Ruby (syntax-wise).

Some helpful resources:

https://github.com/h4cc/awesome-elixir

http://elixirsips.com/