How we upgrade major Rails versions

The core idea behind the way we upgrade Rails revolves around making the application compatible with both the current version and the next version simultaneously. Doing so means that during the upgrade process we're able to boot the application with these versions allowing tests to run against both. This makes it much easier to find regressions by toggling a switch that determines which version to run.This is done by having a Gemfile.next in addition to our current Gemfile. Gemfile.next is simply a symbolic link to the current Gemfile.