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

Author: Matthew Fisher

Makefile

@@ -7,7 +7,7 @@ export GO15VENDOREXPERIMENT=1
 
 # dockerized development environment variables
 REPO_PATH := github.com/deis/${SHORT_NAME}
-DEV_ENV_IMAGE := quay.io/deis/go-dev:0.19.0
+DEV_ENV_IMAGE := quay.io/deis/go-dev:0.20.0
 DEV_ENV_WORK_DIR := /go/src/${REPO_PATH}
 DEV_ENV_PREFIX := docker run --rm -e GO15VENDOREXPERIMENT=1 -v ${CURDIR}:${DEV_ENV_WORK_DIR} -w ${DEV_ENV_WORK_DIR}
 DEV_ENV_CMD := ${DEV_ENV_PREFIX} ${DEV_ENV_IMAGE}