README: update golang requirement to 1.7 (#2412)

The docker engine-api package we use needs golang 1.7+, see:
https://github.com/docker/engine-api/pull/382#issuecomment-244512952

So telegraf won't compile without 1.7

README.md

@@ -43,7 +43,7 @@ Ansible role: https://github.com/rossmcdonald/telegraf
 
 Telegraf manages dependencies via [gdm](https://github.com/sparrc/gdm),
 which gets installed via the Makefile
-if you don't have it already. You also must build with golang version 1.5+.
+if you don't have it already. You also must build with golang version 1.7+.
 
 1. [Install Go](https://golang.org/doc/install)
 2. [Setup your GOPATH](https://golang.org/doc/code.html#GOPATH)