Use npm (instead of Yarn) as default package manager

[tinesoft]

Bug Report or Feature Request (mark with an x)

- [ ] bug report -> please search issues before submitting
- [ x] feature request

Desired functionality.

Yarn is giving me too much trouble, and it's no longer so better as latest npm in speed, consistency and features. For example, it is not working well with nvm.

In fact, when using nvm in a library, the gulp link(npm link) will symlink the project folder to a global node_modules matching the node version being used (for eg: ~/.nvm/versions/node/v8.12.0/lib/node_modules/)

But running yarn install in demo/ app , is not linking to the library in that nvm-node-related global modules, but instead to /usr/local/lib/node_modules/<your-project>.

Mention any other details that might be useful.

• npm option should default to true and save in .yo-rc.json as useYarn key
• config/helpers#installDependencies() should consider that useYarn flag
• update .travis.yml's cache section accordingly

[ghost]

Let me try