Fix migrating db that isn't past squashed migration

[PaarthShah]

Retroactively changing the migration is the only way for this to work: it's a safe operation on its own because:

1. The migration has already been run
2. the operation itself is identical (it was copied from the squashed migration)
3. for a fresh migration, only the squashed version is run anyway

Fixes #303

Migration state after:

easyaudit                                                                                                                                                                        
 [X] 0001_initial                                                                                                                                                                
 [X] 0002_auto_20170125_0759                                                                                                                                                     
 [X] 0003_auto_20170228_1505                                                                                                                                                     
 [X] 0004_auto_20170620_1354_squashed_0019_alter_crudevent_changed_fields_and_more (16 squashed migrations)

[jourdanrodrigues]

My approval doesn't mean anything, sadly. 😬 Hope the maintainers show up soon enough.

[jourdanrodrigues]

There's a need to run poetry lock @PaarthShah

[PaarthShah]

@jourdanrodrigues I'll get to that asap when I'm back at my computer

[PaarthShah]

@jourdanrodrigues poetry lock ran, pushed changes without self-reviewing.

[mschoettle]

Could you bring your branch up to date with master? The lock file is updated and CI now runs with all supported combinations of Python and Django.

Ultimately, it is up to @jheld as the (sole?) maintainer to decide which approach to take (as discussed in #301).

[jheld]

@PaarthShah seems we need another rebase to get rid of the conflicts

[PaarthShah]

@jheld @mschoettle done