20160616103948 CE migration conflicts with EE migration 20160301174731

ZenDesk issue: https://gitlab.zendesk.com/agent/tickets/28675

These two files both try to add an index to the same column:

• EE: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/db/migrate/20160301174731_add_fingerprint_index.rb
• CE: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/db/migrate/20160616103948_add_unique_index_to_keys_fingerprint.rb

If you attempt to upgrade from 8.9.1 CE to 8.9.4 EE, you get a bad error:

Index name 'index_keys_on_fingerprint' on table 'keys' already exists` - `20160301174731_add_fingerprint_index.rb`

I think we may need to check for the existence of this index before inserting.