libVips / slugbuilder support

[evadne]

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 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.

[ddollar]

I don't really use this buildpack any more so I'm just going to accept these changes as they seem reasonable to me.