Ultimately, that's the best solution if you need to collaborate with CC users, but subscribers also get CS6 as part of the package and can install it for free if their OS isn't too new, or can provide you with a .idml file when all else fails.
You can ask the person who sent it to you to save the file as an .idml file, and then resend it. Or better yet, have them package it (File > Package), zip the package folder and send you the package. A CC 2015 package will have the .idml file in it, along with the linked graphics and fonts not included with Typekit.
You can ask the original supplier of that file to Export it as IDML for you. This will allow you to open it, but it will lose (or flatten) anything formatted using features that were new to the updated version.