Use `updated_at` instead of `call_last_updated`

Since this data is only ever pulled in from outside sources, we can use
Rails' generated timestamps instead of a superfluous extra timestamp
column ("call_last_updated"). Since `updated_at` can be directly set in
Rails 4, let's use that attribute instead so that we don't have to store as
much data.

Signed-off-by: David Celis <me@davidcel.is>