Store result ids on ImportResult (postgres only)

[saulm]

I was looking for a solution to https://github.com/activeadmin-plugins/active_admin_import/issues/179

Luckily I'm using postgres, and activerecord-import stores the ids for postgres: https://github.com/zdennis/activerecord-import#return-info

I understand this is not the ideal solution for everyone, but it's consistent with the behavior of the underlying base library.

Let me know if this is something you would like to add or if you need any other changes.

[coveralls]

Coverage decreased (-0.006%) to 97.98% when pulling 54fb15b5095b59bdc299036d81e42965293a4332 on polyglot-solutions:master into 1c7601fcccff35bb4217430e3fd0626312f64f2f on activeadmin-plugins:master.