chore(Makefile): update deis/go-dev to 0.20.0

See https://github.com/deis/docker-go-dev/releases/tag/0.20.0

Author: mboersma

Makefile

@@ -13,7 +13,7 @@ REPO_PATH = github.com/deis/logger
 
 # The following variables describe the containerized development environment
 # and other build options
-DEV_ENV_IMAGE := quay.io/deis/go-dev:0.17.0
+DEV_ENV_IMAGE := quay.io/deis/go-dev:0.20.0
 DEV_ENV_WORK_DIR := /go/src/${REPO_PATH}
 DEV_ENV_OPTS := --rm -v ${CURDIR}:${DEV_ENV_WORK_DIR} -w ${DEV_ENV_WORK_DIR}
 DEV_ENV_CMD := docker run ${DEV_ENV_OPTS} ${DEV_ENV_IMAGE}