Conditionally include ember-get-helper

[cball]

Not needed on 2.0, but it looks like CLI will warn if its included in package.json. Leaving warning for until it either breaks something or we decide to drop 1.13 support.

[BrianSipple]

@cball I could be wrong, but it looks like ember-get-helper already takes care of standing down if it detects that it's being run in a post-1.13 project.

The warning message does, indeed, make things a bit clunky, but at least there's no discernible breakage risk. If it's something that we don't want to deal with, perhaps it would be worth pinging the owner with a PR to make the warning toggle-able.