CONTRIBUTING.md: wrap DCO to 78 chars per line

The DCO is a bit hard to read on Github right now because there is lots
of horizontal scrolling. Reformat it to 78 chars per line like the rest
of the file.

Example: http://imgur.com/LtJEIsl

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Use https to get the latest docker version

To avoid unexpected results since docker was using http.
For instance, my broadband doesn't return not found when it's down but
a html page saying that the internet is down. Docker was showing that
html instead of ignoring it.

Fix #3570

Docker-DCO-1.1-Signed-off-by: Fabio Falci <fabiofalci@gmail.com> (github: fabiofalci)
Fix docs typo: "Gento" -> "Gentoo"

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3587 from tianon/fix-gentoo-typo

Fix docs typo: "Gento" -> "Gentoo"
Merge pull request #3570 from fabiofalci/3569-validate-docker-version

Fetch docker version over https
Un-gofmt ./vendor

We can't be modifying this except to update from upstream.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3534 from philips/reformat-DCO

CONTRIBUTING.md: wrap DCO to 78 chars per line
Merge pull request #3591 from tianon/fix-vendor

Un-gofmt ./vendor