When using a CAS server, the users only have a temporary email
`change@me-foo-cas.com` which can't be changed but by an
administrator.
We need a new environment variable like for SAML to assume the email
from CAS is verified.
* config/initializers/omniauth.rb: define CAS option for assuming
email are always verified.
* .env.nanobox: add new variable as an example.