slugbuilder,slugrunner: Update to cedar-14 stack

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
all: Change from ubuntu to ubuntu-deboostrap image

The cedarish image uses the debootstrap variant, and it doesn’t make
sense to have to versions of the Ubuntu base image kicking around.

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
slugbuilder: Remove non-Heroku buildpacks

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
slugbuilder: Pin buildpack revisions

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
slugbuilder: Silence checkout and cleanup .git directory

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>