Convert Fixnum into String the port number like `db:structure:dump` task in MySQL.

application.rb:

```ruby
module MyApp
  class Application < Rails::Application
    # ...
    config.active_record.schema_format = :sql
  end
end
```

database.yml:

```
development:
  adapter: mysql2
  encoding: utf8
  database: my_app
  username: root
  password:
  port: 12345
```


Run `db:structure:dump` task in MySQL.

output:

```
rake aborted!
no implicit conversion of Fixnum into String
/path/to/myapp/.bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/tasks/mysql_database_tasks.rb:62:in `system'
/path/to/myapp/.bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/tasks/mysql_database_tasks.rb:62:in `structure_dump'
/path/to/myapp/.bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/tasks/database_tasks.rb:142:in `structure_dump'
/path/to/myapp/.bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/railties/databases.rake:288:in `block (3 levels) in <top (required)>'
Tasks: TOP => db:structure:dump
(See full trace by running task with --trace)
```

This solves the issue by convert Fixnum into String the port number.