all: Remove MAINTAINER line from Dockerfiles

It just adds a layer that slows things down.

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>