Deprecation message incorrect for ember-cli-mocha

[blimmer]

In #144 a deprecation warning was added stating that ember-cli-mocha 0.15.0++ can use the new native test helpers.

However, I think that's not quite right. ember-cli-mocha 0.15.0 uses ember-mocha 0.13.x, which does not support the new testing flavors.

https://github.com/ember-cli/ember-cli-mocha/blob/v0.15.0/package.json#L39

I think the issue needed resolved on the ember-cli-mocha side is ember-cli/ember-cli-mocha#259 .

In the meantime, could we update this message and/or silence it for ember-cli-mocha users until they can upgrade?