Replace rubycas-client

Gemfile:
```rb
# TODO: Replace this gem with one that is maintained, like                  
# rack-cas or omniauth-cas
# This is used for course surveys so that professors and TAs/GSIs can log in
# and see their survey results using their Berkeley login
gem 'rubycas-client', git: 'https://github.com/rubycas/rubycas-client'
```