Use machine RC to be able to pick up the boot2docker RC at install time

Signed-off-by: Joffrey F <[email protected]>

Author: shin-

versions

@@ -2,7 +2,7 @@ DOCKER_RELEASE_STAGE=test
 INSTALLER_VERSION=17.06.0-ce-rc2
 DOCKER_VERSION=17.06.0-ce-rc2
 DOCKER_COMPOSE_VERSION=1.14.0-rc2
-DOCKER_MACHINE_VERSION=0.12.0
+DOCKER_MACHINE_VERSION=0.12.0-rc2
 KITEMATIC_VERSION=0.17.0
 VBOX_VERSION=5.1.22
 VBOX_REV=115126