Fix broken migration when upgrading from 2.0.4 to 3.4.1.

Commit 5d0c99f3ad9d46a4ff340d23d4e85e6926e52795 changed the name of the index on rapns_notifications. This can break the upgrade path if the old index name was already used in the db.
Migration rollback should remove the index before dropping a column included in that index.

Either the remove_index call will fail because removing the columns automatically removes its dependent indexes (PostgreSQL) or the remove_column will fail due to the existence of dependent indexes (possibly MySQL?)
Merge pull request #181 from jcoleman/fix-upgrade-migration

Fix migration failures on upgrade and rollback