Fix release.sh sed of install.sh so we preserve tests against 'https://get.docker.io/'

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Symlink vendor/MAINTAINERS over to hack/MAINTAINERS (since it depends explicitly on hack/vendor.sh)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Clean and simplify vendor.sh while making it easier to maintain as well

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3625 from tianon/cleanup-vendor-sh

Clean and simplify vendor.sh while making it easier to maintain as well
Merge pull request #3599 from tianon/fix-install-sed

Fix release.sh sed of install.sh so we preserve tests against 'https://get.docker.io/'
Merge pull request #3624 from tianon/vendor-maintainers

Symlink vendor/MAINTAINERS over to hack/MAINTAINERS