Added a way to authenticate github api calls for composer

There is 2 great news with this feature:

- we are not restricted by the low github API rate limit when not being authenticated (the restriction is by IP so it could be hard on heroku build servers as you would be impacted by other projects as well). Recent composer versions are using the source install as fallback when they cannot download the dist instead of failing directly, but the source is not cached between deployments in this buildpack
- if your project needs to depend on some other private github repos, it is now possible to install the dependencies assuming your token has access to your private repos