Problem

You are trying to upgrade one of your gems (ie uglifier) and at the same time you are using a new gemset (ie for rails 4.2.4), and you get the following error when you are trying to bundle:

Could not find libv8-3.16.14.9 in any of the sources

Solution

Upgrade to rails 4.2.4 first and bundle and afterwards change the other gem and do bundle again.