Rename overlayfs to overlay

Since Linux 3.18-rc6, overlayfs has been renamed overlay.

This change was introduced by the following commit in linux.git:
ef94b1864d1ed5be54376404bb23d22ed0481feb ovl: rename filesystem type to "overlay"

Signed-off-by: Lénaïc Huard <lhuard@amadeus.com>

Docker-DCO-1.1-Signed-off-by: Lénaïc Huard <lhuard@amadeus.com> (github: jfrazelle)
Rename the overlay storage driver

so that docker is started with `docker -d -s overlay` instead of `docker -d -s overlayfs`

Signed-off-by: Lénaïc Huard <lhuard@amadeus.com>

Docker-DCO-1.1-Signed-off-by: Lénaïc Huard <lhuard@amadeus.com> (github: jfrazelle)
Remove the last references to overlayfs

This only renames docker internal structures.
It has no impact on the end-user.

Signed-off-by: Lénaïc Huard <lhuard@amadeus.com>

Docker-DCO-1.1-Signed-off-by: Lénaïc Huard <lhuard@amadeus.com> (github: jfrazelle)
Bump version to v1.4.0

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