Add dependency lint

[ryanto]

We need to be sure we're getting the right dependency versions on travis. We've solved this problem in other projects with dependency lint.

[ryanto]

I took a shot at this earlier and it looks like were still getting conflicts around ember data. The build has the right version, but dependency-lint notices two data versions between addon docs and storefront. So I'm misunderstanding something here.

 ~/p/ember-data-storefront $ ember dependency-lint

ember-data
Allowed: (any single version)
Found: 3.4.0, 3.1.1
ember-data-storefront
├── ember-data@3.1.1
└─┬ ember-cli-addon-docs
  └── ember-data@3.4.0

I'm unsure how to resolve this. I commented on a discuss thread about a similar issue. My guess is ember-try + yarn resolutions, but I'm not sure.

I'm blocked on this until I can understand what's going on a bit better.