Use reasonable defaults based on the project

[jelhan]

1. Use yarn or NPM depending on presence of either yarn.lock or package-lock.json.
2. Use minimum node version listed as supported in engines.node key of package.json.
3. Run all scenarios defined in config/ember-try.js.
4. Run against all browsers listed in testem's launch_in_ci configuration (testem.js).

Closes #32