Description:
    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, it will:
    - checkout a "jhipster_upgrade" branch
    - generate a application upgrade reference
    - apply the reference point to your application
    - generate the upgraded application
    - switch back to your original branch and merge the upgrade branch into it
