Rubocop reports different offenses between CI and local dev environment

### Problem
I noticed 7 offenses when running Rubocop in my local dev environment, but those are not reported in CI.  I opened !921 in order to fix the local offenses (by disabling the offending cop `Rails/BulkChangeTable`), but then Rubocop on CI reports `Unnecessary disabling of Rails/BulkChangeTable`.  It appears that Rubocop is running differently on CI (different gem version? different configuration?) from the local dev environments.  A few other engineers reported the same behavior so it doesn't seems to be a problem with a single dev environment.

**Local Dev Rubocop Results:**
```
➜  customers-gitlab-com git:(staging) be rubocop
Inspecting 494 files
.........................................................................................................................................................................CC.C...........C........C.C..........................................................................................................................................................................................................................................................................................................

Offenses:

db/migrate/20160408050701_add_devise_to_customers.rb:3:5: C: Rails/BulkChangeTable: You can combine alter queries using bulk: true options.
    change_table(:customers) do |t|
    ^^^^^^^^^^^^^^^^^^^^^^^^
db/migrate/20160408063635_add_oauth_columns_to_customers.rb:3:5: C: Rails/BulkChangeTable: You can use change_table :customers, bulk: true to combine alter queries.
    add_column :customers, :provider, :string # rubocop:disable Migration/AddLimitToStringColumns
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
db/migrate/20160426212810_add_country_state_city_and_zip_code_to_customers.rb:3:5: C: Rails/BulkChangeTable: You can use change_table :customers, bulk: true to combine alter queries.
    add_column :customers, :country, :string # rubocop:disable Migration/AddLimitToStringColumns
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
db/migrate/20171228073933_add_extra_info_fields_to_orders.rb:3:5: C: Rails/BulkChangeTable: You can use change_table :orders, bulk: true to combine alter queries.
    add_column :orders, :gl_namespace_id, :string # rubocop:disable Migration/AddLimitToStringColumns
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
db/migrate/20190206170351_add_confirmable_to_devise.rb:3:5: C: Rails/BulkChangeTable: You can use change_table :customers, bulk: true to combine alter queries.
    add_column :customers, :confirmation_token, :string # rubocop:disable Migration/AddLimitToStringColumns
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
db/migrate/20190206170351_add_confirmable_to_devise.rb:14:5: C: Rails/BulkChangeTable: You can use change_table :customers, bulk: true to combine alter queries.
    remove_columns :customers, :confirmation_token, :confirmed_at, :confirmation_sent_at
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
db/migrate/20190512230632_add_address1_and_address2_to_customers.rb:3:5: C: Rails/BulkChangeTable: You can use change_table :customers, bulk: true to combine alter queries.
    add_column :customers, :address_1, :string # rubocop:disable Migration/AddLimitToStringColumns
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

494 files inspected, 7 offenses detected
```

**CI Rubocop Results:**
```
$ bundle exec rubocop
 Inspecting 494 files
 ..............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
 494 files inspected, no offenses detected
```

### Proposal
Figure out why Rubocop is running differently on CI and make it consistent to avoid discrepancies.

### Result
Dependable Rubocop results in local dev and CI.

### Next steps (if any)

### How will we measure success?

Story points: 2.0