Fix travis badge URL

Repository moved from dotcloud to docker

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

Docker-DCO-1.1-Signed-off-by: Ben Firshman <ben@firshman.co.uk> (github: bfirsh)
Merge pull request #283 from bfirsh/fix-travis-badge

Fix travis badge URL