fix undefined method `shift' error on migration

Fixes an error I was having during the 6.0 -> 7.1 migration on a long running GitLab instance:

```
undefined method `shift' for #<Issue::ActiveRecord_Relation:0x007f29837d1050>
```

I got the solution from https://gitlab.com/gitlab-org/gitlab-ce/issues/380.

I'm unsure how I can check to make sure this is completely correct and that the migration went as expected however I am reloading my 6.0 install to make sure the data between the 6.0 and 7.1 install appears correct.

Looks like this should be ported back to each of the following so that users don't have errors during the upgrade to 7.1:

v7.1.1 v7.1.0 v7.0.0 v6.9.2 v6.9.1 v6.9.0
@randx any further feedback? Would like to get this fixed for other going through upgrade process.