Rate-Limiting

Separate CircleCI builds can share the same IP. This can cause certain services, such as Github, to enable rate-limiting.

See Composer's documentation for specifically creating tokens on Github:
https://getcomposer.org/doc/articles/troubleshooting.md#api-rate-limit-and-oauth-tokens

Then store the token as an environment variable in the CircleCI dashboard under Project Settings > Environment Variables and set it before the composer install run step.