libVips / slugbuilder support

I do not know much about Apt, but the following changes seem to work, and without these changes certain things are impossible.

* Packages like libvips [have their compiled libraries under `/usr/lib/:architecture`](https://packages.debian.org/sid/i386/libvips-dev/filelist) instead of just `/usr/lib`.
  * This seems to affect `LD_LIBRARY_PATH` and `LIBRARY_PATH`
* Certain gems like `ruby-vips` need pkg-config to work in order to compile native extensions, which in turn require it to be defined.
  * While this variable seems to be defined in Cedar 14, Apt-specific directories were not listed
* Certain packages like libpango, libpangocairo and libpangoft2 all need to be compiled in the same way; as Cedar 14 already has one of them, without the `--reinstall` flag the artifacts will conflict.
  * See https://github.com/evadne/heroku-buildpack-apt/commit/4d898d55ea952ccb3962daf291bdf248644275d0

Please consider this pull request. Thanks again.