Strategy: composer

Experimental implementation for composer. 

see: 
- [Composer hitting GitHub API rate limits - CircleCI](https://circleci.com/docs/composer-api-rate-limit)
- [composer/troubleshooting.md at 31eadc6920cd1866bc061fb0087798c37e2b7d14 · composer/composer](https://github.com/composer/composer/blob/31eadc6920cd1866bc061fb0087798c37e2b7d14/doc/articles/troubleshooting.md)