Remove Ubuntu 15.04 (Vivid) from DEB build as it will be EOL

Ubuntu 15.04 (Vivid) will be EOL'd in January,
so we should remove it from our builds in the
Docker 1.10 release.

For information about the EOL data, see:
https://wiki.ubuntu.com/Releases

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>