Update vendored gravity version. (#261)

Installing 5.5.20 fails due to:

  gravitational/gravity#1992

Lets update this to something more contemporary.

Author: wadells

docker/Makefile

@@ -19,7 +19,7 @@ DOCKER_VERSION ?=
 DOCKER_TAG ?=
 DOCKER_ARGS ?= --pull
 
-GRAVITY_VERSION := 5.5.20
+GRAVITY_VERSION := 5.5.50
 TERRAFORM_VERSION := 0.12.9
 CHROMEDRIVER_VERSION := 2.39
 TERRAFORM_PROVIDER_AWS_VERSION := 1.19.0