Problem: migration 0004 breaks upgrade if fprules have been customized

[jhsimpson]

Migration 0004 runs loaddata, using pronom_88.json. That json file has new fprules defined in it, for example:

    {
        "fields": {
            "count_not_okay": 0,
            "replaces": null,
            "count_attempts": 0,
            "uuid": "dfc4197d-d265-4eac-839d-b267d4ac4a9f",
            "format": "dc76c66b-8be8-45f4-a8c8-7d40a47a5b52",
            "lastmodified": "2016-12-14T14:04:19Z",
            "enabled": true,
            "command": "80314f3c-5d48-4ad1-a9f2-2c0f7c7b229d",
            "purpose": "characterization",
            "count_okay": 0
        },
        "model": "fpr.fprule",
        "pk": 712
    },

Each new instance of the fprule model defines a pk to use. If the database already has a row with that pk used, then the loaddata method tries to overwrite it. If that row has a uuid that is referred to in a replaces_id property of some other row in the database, the upgrade fails on an integrityError.

It is not necessary to pre-define the pk values. Omitting them works, the database will assign new keys automatically, and avoid this problem.

It would be better to avoid the use of loaddata altogether, ( see #38 ) but removing the pk's will fix the problem of not being able to upgrade Archivematica from 1.5.1 to 1.6.0 when the fpr has been customized.

[jhsimpson]

fixed in #47