Fails to build when GOOS=plan9

[mholt]

This package is an indirect dependency of Caddy 2, and is preventing Caddy 2 from being available for Plan 9:

# github.com/chzyer/readline
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/operation.go:234:4: undefined: ClearScreen
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/readline.go:129:20: undefined: GetScreenWidth
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/readline.go:132:22: undefined: DefaultIsTerminal
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/readline.go:142:26: undefined: DefaultOnWidthChanged
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/remote.go:324:2: undefined: DefaultOnWidthChanged
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/remote.go:346:17: undefined: GetScreenWidth
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/remote.go:362:16: undefined: DefaultIsTerminal
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/terminal.go:51:2: undefined: SuspendMe
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/utils.go:81:29: undefined: State
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/utils.go:241:9: undefined: State
../../../../../go/pkg/mod/github.com/chzyer/readline@v0.0.0-20180603132655-2972be24d48e/terminal.go:51:2: too many errors

Some tweaking of build tags should be able to help solve the problem.

Downstream issue: caddyserver/caddy#3615