Resolve configuration file path on migration

It changes so that the path of the migration config file is resolved before it is required.
This avoids `Cannot find module` error when the configuration file path is specified as relative path:

```bash
$ cd some-project/
$ cat db/config.js
// Sample Config File
module.exports = {

  directory: './migrations-folder',

  database: {
    client: 'sqlite3',
    connection: {
      filename: './db.sqlite3'
    }
  }

};
$ ./node_modules/.bin/knex migrate:make create_posts -c ./db/config.js
```