chore(builder): migrate to cedar-14 stack

With quantal coming out-of-support, heroku implemented the cedar-14
stack. This stack is what's used on heroku in production today AFAIK.
Migrating over to this stack makes us more compatible with Heroku.
ref(slugbuilder): remove third party buildpacks; bump version

In order to stay as close to Heroku's stack, removing third party
buildpacks that can later specified with BUILDPACK_URL helps remove
the maintenance burden on ourselves and the buildpack maintainers.
Merge pull request #1975 from bacongobbler/cedar-14

chore(builder): migrate to cedar-14 stack