Dashboard: add gevent to requirements
Dashboard: add gevent to requirements

This enables us to use `--worker-class gevent` in Gunicorn which is desirable
if we want to have our Gunicorn workers doing other work when they're blocked
on I/O bound operations.

artefactual/archivematica,artefactual/archivematica,artefactual/archivematica,artefactual/archivematica