readme: Update minimum version to build from source (#2024)

Re: #2009, 1.9 or newer is needed because of the introduction of `sync.Map`

README.md

@@ -59,7 +59,7 @@ customize your build in the browser
 pre-built, vanilla binaries
 
 ## Build
-To build from source you need **[Git](https://git-scm.com/downloads)** and **[Go](https://golang.org/doc/install)** (1.8 or newer). Follow these instruction for fast building:
+To build from source you need **[Git](https://git-scm.com/downloads)** and **[Go](https://golang.org/doc/install)** (1.9 or newer). Follow these instruction for fast building:
 
 - Get source `go get github.com/mholt/caddy/caddy` and then run `go get github.com/caddyserver/builds`
 - Now `cd` to `$GOPATH/src/github.com/mholt/caddy/caddy` and run `go run build.go`