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

@@ -2,7 +2,7 @@ SHORT_NAME := minio
 
 # dockerized development environment variables
 REPO_PATH := github.com/deis/${SHORT_NAME}
-DEV_ENV_IMAGE := quay.io/deis/go-dev:0.17.0
+DEV_ENV_IMAGE := quay.io/deis/go-dev:0.20.0
 DEV_ENV_WORK_DIR := /go/src/${REPO_PATH}
 DEV_ENV_PREFIX := docker run --rm -v ${CURDIR}:${DEV_ENV_WORK_DIR} -w ${DEV_ENV_WORK_DIR}
 DEV_ENV_CMD := ${DEV_ENV_PREFIX} ${DEV_ENV_IMAGE}