Drop support for Ember < 3.8

[SergeAstapov]

@marcoow wonder if you are up to this change, which indeed is breaking and may cause a churn but removes extra deps from the addon.

[marcoow]

At some point sure but I'd see whether there are other things we want to merge before a major version change

[SergeAstapov]

@marcoow we can do #732 if you'd like for the next major release. OR you may cut a major (let's say 0.6.0) and followed by v2 conversion which will be another major release.

Personally I don't have preference so would delegate to you as maintainer of the library.

[SergeAstapov]

@marcoow as we already did breaking changes in #747 and #748, do you think we can land this one as well and cut a new release?

[marcoow]

yep 👍