Install dependencies in start-app.sh

This shell script is used by the pp-development directory to start the app.
When an external user runs this script for the first time, we should install
dependencies for them rather than exploding. This will be a no-op if the
deps are installed already.
Remove instruction to install supervisor

This is handled by npm install as of d959a62c072935bb6f6657c902f8f5e318b192af.
Merge pull request #185 from alphagov/improve-start-app

Install dependencies in start-app.sh