Merge pull request #183 from avalonmediasystem/hotfix/gem-locking

Lock hydra-head and devise (-> master)
Replace Hydra::Migrate with simpler VersionableModel concern and regular ActiveModel migrations
R2 -> R3 migrations
Let the MediaObject callbacks handle what they can
Improve migration code (with feedback and benchmarks)
update with better styling
Roll all R2->R3 related model changes into a single migration
Merge pull request #287 from atomical/task/update-interface

update with better styling
Skip batches with unreadable manifests without bailing out
Merge pull request #288 from avalonmediasystem/bugfix/vov-2538

Skip batches with unreadable manifests without bailing out
Merge pull request #286 from avalonmediasystem/feature/vov-2448

R3 migrations
Remove test for nonexistent migration
Revert "Remove test for nonexistent migration"

This reverts commit 14e36c3fc61af639e87f23438adf4527d5cd5fdd.
Restore and fix R3 rights metadata migration spec; add more fixtures for future migration tests
Account for nonexistent matterhorn workflow in R2->R3 migration
Make sure derivative file exists before asking Mediainfo to load it
Bump mediaelement_rails to include fix for VOV-2582
Merge pull request #289 from avalonmediasystem/bugfix/VOV-2582

Bump mediaelement_rails to include fix for VOV-2582
Fix access control context in batch for "hidden" attribute
Merge pull request #290 from avalonmediasystem/bugfix/VOV-2587

Fix access control context in batch for "hidden" attribute