Cleanup requirements

Cleanup requirements

* python-dateutil is only used in tests, so remove it from the
  production requirements
* mysqlclient is prefered to MySQLdb, so use that [1]
* selenium does not have its version fixed, so resolve that

[1] https://docs.djangoproject.com/en/1.8/ref/databases/#mysql-db-api-drivers

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>