Automated updates

[ericmann]

Presently, when new versions of the server app are available, the upgrade path looks something like:

• Turn off Vagrant
• cd into the app directory within the project
• Update the project
• npm install --no-bin-link to pull in additional dependencies
• Restart Vagrant

Unfortunately, this upgrade path will break when we move the server app to a globally-installed module with 10up/Derrick#12

We need the app to be capable of updating itself, ideally when triggered externally.

[ericmann]

When the server starts, it can automatically run a version check (ala Composer) and alert the end user if an update is available or just update itself.

A "started" hook in the server app would also allow for easy integration with and control of the Docker daemon in #5.