This is a Heroku buildpack for the libsodium library.
See Using Multiple Buildpacks for an App In brief:
heroku buildpacks:add --index 1 https://github.com/<user>/heroku-buildpack-libsodium.git
If you use the RbNaCl Ruby library, make sure to check out the rbnacl-libsodium Ruby Gem to compile libsodium.