Add explicit STACK check

[edmorley]

This buildpack currently doesn't validate whether it supports the current $STACK.

This means if someone pins to an old SHA of the buildpack or forks the buildpack, then later a new stack is introduced, they get an error like cp: cannot stat 'bin/nginx-heroku-18': No such file or directory (in the case of older releases), or more recently they'd get a tar error.

Example support ticket:
https://heroku.support/985441

Whilst nothing can be done about older releases now, it would be good to add an explicit stack check to make this failure mode more obvious in the future :-)