Since Rails 5.1 missing migration version results in following error:
```
StandardError: Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for:
```
This PR fixes all migration files.
POST /api/statuses Params: status (text contents), in_reply_to_id (optional)
GET /api/statuses/:id
POST /api/statuses/:id/reblog
GET /api/accounts/:id
GET /api/accounts/:id/following
GET /api/accounts/:id/followers
POST /api/accounts/:id/follow
POST /api/accounts/:id/unfollow
POST /api/follows Params: uri (e.g. user@domain)
OAuth authentication is currently disabled, but the API can be used with HTTP Auth.
