Improve update_requirements

* Create a sha1 file of REQUIREMENTS and REQUIREMENTS_SERVER and use it to
check wether the files have changed or not.
* In case something has changed install everything as wheel first, then move
the whole virtualenv from env to env_old, create a new virtualenv and install
using --no-index from the previously created wheels.
* Use the project name as prompt prefix so it's clear which virtualenv is
active while working on a webserver.