Description:
    Updates JHipster version, and upgrades generated application

Example:

    jhipster upgrade

    Assuming you have Git installed, and you run this command in the Git branch you are using for development, JHipster Upgrader will :
    - checkout a "jhipster_upgrade" branch
    - update generator-jhipster via npm
    - regenerate your whole application
    - switch back to your development branch, and start the merge
