Fixes #24122 - Remove limit from LookupValue value
Fixes #21100 - set DB migration compatibility to 4.2
Put all existing DB migrations into 4.2 compatibility mode as they don'tspecify null => true on timestamp columns etc.
Fixes #10409 - Rails 4 compatible changes with Rails 3
fixes #1964 lookup_keys.default_value can hold > 255 characters