Update required golang version in Readme

Fixes #4616.

README.md

@@ -51,7 +51,7 @@ https://www.consul.io/docs
 ## Developing Consul
 
 If you wish to work on Consul itself, you'll first need [Go](https://golang.org)
-installed (version 1.9+ is _required_). Make sure you have Go properly installed,
+installed (version 1.10+ is _required_). Make sure you have Go properly installed,
 including setting up your [GOPATH](https://golang.org/doc/code.html#GOPATH).
 
 Next, clone this repository into `$GOPATH/src/github.com/hashicorp/consul` and