I moved the setting of the config vars from the YAML hash in the release script to the .profile.d script. This should fix the problem that the config vars, including the PATH, might not be set if the buildpack is used together with other buildpacks (see issue #3 which would be fixed with this).
Setting config vars in the release script also seems to be deprecated, as it is not mentioned here https://devcenter.heroku.com/articles/buildpack-api#buildpack-api (under bin/release), and here https://devcenter.heroku.com/articles/buildpack-api#default-config-values it is advised to use a .profile.d script for adding default config values.
Set PATH and other vars in .profile.d script
This fixes the problem that the config vars (including PATH) might not
be set if the buidpack is used together with other buidpacks.