Builder shouldn't crash if there is no bin/release file

While adding support for a cache dir in dokku I found out that it is possible for buildpacks to have no `bin/release` file. See for example the `diet` (adds caching support) branch on the official heroku nodejs buildpack: https://github.com/heroku/heroku-buildpack-nodejs/tree/diet/bin. I even found a comment stating that `bin/release` might go away soon: https://github.com/heroku/heroku-buildpack-nodejs/pull/52#commitcomment-4313482.

This PR adds some resilience for a missing `bin/release` file.