Better handling of Rails upgrades.

[rywall]

If Rails changes attribute types, we are then unable to instantiate our snapshots. See collectiveidea/audited#289 for a similar issue. I've worked around this by rebuilding all the snapshots but perhaps there is a better solution that doesn't involve serializing the database column types.

[rywall]

Also had to work around this issue in https://github.com/culturecode/connect_engine/commit/c58f1c1c6629dfe314d4a5189fb2a9da2d2b36cf