Merge pull request opencontainers#158 from q384566678/makefile-fix

Add go version requirement

Author: Zhou Hao

Makefile

@@ -1,5 +1,3 @@
-GO15VENDOREXPERIMENT=1
-export GO15VENDOREXPERIMENT
 PREFIX ?= $(DESTDIR)/usr
 BINDIR ?= $(DESTDIR)/usr/bin
 

README.md

@@ -1,6 +1,7 @@
 # oci-image-tool [![Build Status](https://travis-ci.org/opencontainers/image-tools.svg?branch=master)](https://travis-ci.org/opencontainers/image-tools)[![Go Report Card](https://goreportcard.com/badge/github.com/opencontainers/image-tools)](https://goreportcard.com/report/github.com/opencontainers/image-tools)
 
 `oci-image-tool` is a collection of tools for working with the [OCI image format specification](https://github.com/opencontainers/image-spec).
+To build from source code, image-tools requires Go 1.7.x or above.
 
 ## Install