This is a Heroku buildpack that shadows the system-provided curl executable and libcurl with a custom built version, most notably for its HTTP/2 support – provided by nghttp2.
It is not meant to be used standalone, just as an extra layer before the actual language buildpack.
See Heroku's documentation on multiple buildpacks.
Example for a Vapor app:
$ heroku buildpacks:set https://github.com/vapor-community/heroku-buildpack.git
$ heroku buildpacks:add https://github.com/vzsg/heroku-buildpack-curl-http2.git --index 1
$ git push heroku master
...
You can verify the installation using the following command:
$ heroku run "curl --version"