docker-resin-supervisor-disk: Update to v7.1.20

Brings in the following improvements since 7.1.18:

- Replace the gosuper component with a node module that handles communication with systemd, and stop using an init system in the supervisor container
- Remove any leftover knex migrations locks before running migrations

Change-type: patch
Changelog-Entry: Update supervisor to v7.1.20
Signed-off-by: Akis Kesoglou <akis@resin.io>

Author: dfunckt

meta-resin-common/recipes-containers/docker-disk/docker-resin-supervisor-disk.bb

@@ -10,7 +10,7 @@ SUPERVISOR_REPOSITORY_x86 = "resin/i386-supervisor"
 SUPERVISOR_REPOSITORY_x86-64 = "resin/amd64-supervisor"
 SUPERVISOR_REPOSITORY_intel-quark = "resin/i386-nlp-supervisor"
 
-SUPERVISOR_TAG ?= "v7.1.18"
+SUPERVISOR_TAG ?= "v7.1.20"
 TARGET_REPOSITORY ?= "${SUPERVISOR_REPOSITORY}"
 TARGET_TAG ?= "${SUPERVISOR_TAG}"
 LED_FILE ?= "/dev/null"