Restore support for Ember 2.x

[mixonic]

master of the vertical-collection repo had some Ember-pre-3.x
compatibility stuff removed back in 2019. For example:

• 011d9a4

Restore one of those changes.

[mixonic]

As @rwwagner90 reviewed #321 he noted that the test suite was not running for Ember 2.x. I replied that it seemed like the 2.x test suite was just broken, indeed master generally is in a poor state.

Despite there being some 1.13 compatibility code we found and dropped as part of #321, there was also an earlier effort to drop some compatibility code from master, for example in #289 (as pointed out by @rwwagner90 over DM). Based on us still finding compat code on master, I had presumed that master still should support 2.x, but I was wrong!

In this PR I've restored the Ember 2.x compatibility code and massaged the ember try config until we have a suite passing through all of 2.x.

[RobbieTheWagner]

Do we need this in ember-try since it's a dependency of the addon already now?

[mixonic]

Yes, this is required. I confirmed it by dropping it locally and running the try config.

The dependency in the package.json is ^1.2.1, where this is 1.2.1. Wait wait, I agree, the resolutions should take care of that, however practically it doesn't do the trick on its own.