Checksum_audit_logs updated to account for changes in Fedora 4.

Notice that ChecksumAuditLog.prune_history now expects a pid rather than a version as the parameter.

In addition, Audit.logs() now does not expect a parameter.

Conceptually, the bigger change is that now ChecksumAuditLog will NOT include the datastream ID when it saves the data. In the pass, the datastream IDs was a parameter (although it was always 'content') and now we are not passing this parameter. The MySQL table checksum_audit_logs remains unchanged. We could later on update this code to handle multiple datastreams if we want to.
This line and the prior can now be combined since we don't seem to use `@version` anywhere:

```ruby
@version_id = @f.datastreams['content'].versions.first.to_s.split('/').last
```