Remove unused Engine.Logf

The `Engine.Logf` method was unused and confusing.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

Docker-DCO-1.1-Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com> (github: jfrazelle)
Bump version to v1.4.0

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)