Update mysql_database_tasks.rb

Fix an error if the mysql port is provided in num like this:

```shell
no implicit conversion of Fixnum into String
activerecord-4.0.0/lib/active_record/tasks/mysql_database_tasks.rb:62:in `system'
activerecord-4.0.0/lib/active_record/tasks/mysql_database_tasks.rb:62:in `structure_dump'
activerecord-4.0.0/lib/active_record/tasks/database_tasks.rb:142:in `structure_dump'
activerecord-4.0.0/lib/active_record/railties/databases.rake:288:in `block (3 levels) in <top (required)>'
```