[emcniece]

This PR updates the documentation to reflect the requirement of Golang v1.10, up from v1.8.

A recent attempt to build the project resulted in some errors:

$ go version 
go version go1.9.4 darwin/amd64

$ go build -v -ldflags '-s' -o ../../checkup
github.com/miekg/dns
# github.com/miekg/dns
../../../github.com/miekg/dns/dnssec_keyscan.go:290:7: undefined: strings.Builder
../../../github.com/miekg/dns/msg_helpers.go:271:8: undefined: strings.Builder
../../../github.com/miekg/dns/serve_mux.go:43:9: undefined: strings.Builder
../../../github.com/miekg/dns/types.go:440:10: undefined: strings.Builder
../../../github.com/miekg/dns/types.go:464:10: undefined: strings.Builder
../../../github.com/miekg/dns/types.go:492:10: undefined: strings.Builder
../../../github.com/miekg/dns/types.go:513:29: undefined: strings.Builder
../../../github.com/miekg/dns/types.go:523:28: undefined: strings.Builder

Searching miekg/dns revealed miekg/dns#940 in which the author recommends upgrading Go versions. This repo also notes that it aims to support the latest 2 versions of Go, which are currently 1.12 and 1.11.

Version 1.10 is the next closest version that builds successfully.