Support composer version specify

Composer's `self-update` & getcomposer's `install` can specific version of composer.phar

Hope to support that :)

(I don't like that the revision of composer.phar may change depending on the time due to the fact that task `composer:get` is called.)

#### References

* [Download Composer](https://getcomposer.org/download/)
* [Documentation 13. self-update](https://getcomposer.org/doc/03-cli.md#self-update)