Now including antivirus tests on Travis.
Fixed bug in AF::Datastream decorator
Removed `installed?' method from Antivirus module
Factored out FileContentDatastream tests into new spec module
Load antivirus service in DulHydra initializer
Added clamav gem dependencies
Antivirus scan implemented in DulHydra::HasContent concern in around_save callback
Virus check preservation event created after save
Closes #839
Fixed spec test
Merge pull request #858 from duke-libraries/839-vc-content-file-before-save

839 vc content file before save