Data race about promptui

[hallazzang]

FYI, on my machine I get:

==================
WARNING: DATA RACE
Read at 0x00c420010160 by main goroutine:
  github.com/manifoldco/promptui.(*Prompt).Run()
      /Users/halla/Documents/Develop/Golang/src/github.com/manifoldco/promptui/prompt.go:197 +0x7b1
  main.main()
      /Users/halla/Develop/Golang/src/github.com/hallazzang/tests/promptui-issue55/main.go:25 +0x112

Previous write at 0x00c420010160 by goroutine 17:
  github.com/manifoldco/promptui.(*Prompt).Run.func2()
      /Users/halla/Documents/Develop/Golang/src/github.com/manifoldco/promptui/prompt.go:139 +0xa7b
  github.com/chzyer/readline.(*DumpListener).OnChange()
      /Users/halla/Documents/Develop/Golang/src/github.com/chzyer/readline/operation.go:516 +0x96
  github.com/chzyer/readline.(*Operation).ioloop()
      /Users/halla/Documents/Develop/Golang/src/github.com/chzyer/readline/operation.go:339 +0x771

Goroutine 17 (running) created at:
  github.com/chzyer/readline.NewOperation()
      /Users/halla/Documents/Develop/Golang/src/github.com/chzyer/readline/operation.go:88 +0x7f6
  github.com/chzyer/readline.(*Terminal).Readline()
      /Users/halla/Documents/Develop/Golang/src/github.com/chzyer/readline/terminal.go:95 +0x53
  github.com/chzyer/readline.NewEx()
      /Users/halla/Documents/Develop/Golang/src/github.com/chzyer/readline/readline.go:167 +0x62
  github.com/manifoldco/promptui.(*Prompt).Run()
      /Users/halla/Documents/Develop/Golang/src/github.com/manifoldco/promptui/prompt.go:114 +0x1e9
  main.main()
      /Users/halla/Develop/Golang/src/github.com/hallazzang/tests/promptui-issue55/main.go:25 +0x112

When I run the basic example using following commands:

$ go build -race main.go  # In this step, there's no output
$ go run -race main.go  # It spits out the message above