Remove direct dependency on ember-template-lint

Thanks for @rwjblue for suggesting this approach.

eslint-plugin-hbs had a direct dependency on ember-template-lint but with the release of ember-template-lint@1.0.0, having our own version as a direct dependency of eslint-plugin-hbs meant that this eslint plugin cannot interoperate with either ember-template-lint@1 or ember-template-lint@0.8.

We therefore have moved ember-template-lint into both peerDependencies and devDependencies and we will throw an error if it cannot be imported.