Incorporate changes from jdob's comments on my fix for package category
imports.
Add a symlink to run migration 0004 again as 0006.

I modified 0004 to handle the case where the change it is making
(changing repo_id's on package groups to match the repo they are
associated with) results in a DuplicateKeyError. To handle this case, we
simply remove the offending package group since there is already one
that represents what it would become.
Update a docblock.
Handle DuplicateKeyErrors by removing the duplicate unit from the repo
and decrementing the unit count.

Added another unit test to verify correct handling when a unit is in two
repositories.
Merge pull request #156 from pulp/rbarlow_add_pkg_group_repo_id_migration

Rbarlow add pkg group repo id migration