fix(builder): exec the runner
fix(builder): Non-root slugrunner and slugbuilder

This improves the compatibility to heroku and stops running apps as root
fix(builder): Reverted to default php buildpack

The pervious compatibility fixes made the heroku php build pack work
out-of-the-box again
fix(controller): Use correct entrypoint for run

Running commands in an image build using git push requires to use
‘/runner/init’ as entrypoint, otherwise environment variables inject by
buildpacks are missing.

Fixes #1801, fixes #1568
Merge pull request #1972 from johanneswuerbach/buildpack-env

fix(builder): Non-root slugrunner and slugbuilder
docs(installing_deis): remove xip.io instructions for ELB

xip.io simply doesn't work for ELBs - it returns an IP of
54.64.0.0 which obviously isn't a valid IP.

I suspect it works for CNAMEs in general, but not for CNAMEs
which resolve to multiple IPs, as ELBs do.
Merge pull request #1985 from johanneswuerbach/run-env

fix(controller): Use correct entrypoint for run
Merge pull request #2003 from carmstrong/remove_xip_io

docs(installing_deis): remove xip.io instructions for ELB
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.