FetchAdapter deprecation warning after recent frontend version bumps

[jtgeibel]

Current master is emitting the following warning one or more times for many of the tests.

WARN: DEPRECATION: FetchAdapter is deprecated, it is no longer required for ember-data>=3.9.2 [deprecation id: deprecate-fetch-ember-data-support]

It's confusing to me that ember-data>=3.9.2 is mentioned, when we are still on ember-data=2.18.2. (#1990 has already been opened to upgrade ember-data, but that PR is failing)

This warning appears to have been introduced in #1977. (See https://travis-ci.com/rust-lang/crates.io/jobs/268668867 for the full log.)

[jtgeibel]

cc @Turbo87

[Turbo87]

part of the issue was caused by ember-cli/ember-fetch#391. by now we are on ember-data@3.15, but ember-fetch is still showing the deprecation because I haven't resolved the deprecation yet. I'll have a look at that tomorrow :)