added missing extension.

Fixed a typo. Without giving an extension the command `rails g cas_session_store_migration` has resulted in creating the migration without the ruby extension so that the file was skipped by `ActiveRecord::Migration`

I am using Rails4.1 and Ruby2.1:

```zsh
nirnanaaa@Magmaw ~/Documents/CASApp $ rails g cas_session_store_migration                                                                                         
   create  db/migrate/20140417170814_create_rack_cas_sessions.rb
```