Update to current standard DCO 1.1

The previous version was an IBM-modified one from LXC. Instead let's use
the standard DCO 1.1 as it currently is on developercertificate.org.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Merge pull request #2 from stgraber/master

Update to current standard DCO 1.1