support for the new Cedar-14 stack

This PR adds support for the new Cedar stack Heroku is currently rolling out. The version of Lua previously included in this buildpack was compiled using `libreadline5-dev` which is not included in the new stack (see here: https://devcenter.heroku.com/articles/cedar-ubuntu-packages).  

I have added a new Lua binary compiled using `libreadline6-dev`. Both versions of Lua are included and the right one is chosen during the compile step based on the `$STACK` env variable.