Send update-check message to stderr

[briandealwis]

Our new-version message is sent to stdout and so is mixed up with command output, as noted in #3582 (comment):

skaffold/cmd/skaffold/app/cmd/cmd.go

Lines 115 to 116 in 5a5c603

	case msg := <-updateMsg:
	fmt.Fprintf(cmd.OutOrStdout(), "%s\n", msg)

We should be sending the new-version message to stderr, just like our survey prompt:

skaffold/cmd/skaffold/app/cmd/cmd.go

Lines 123 to 124 in 5a5c603

	if err := survey.New(opts.GlobalConfig).DisplaySurveyPrompt(cmd.OutOrStdout()); err != nil {
	fmt.Fprintf(cmd.OutOrStderr(), "%v\n", err)

[codecov]

Codecov Report

Merging #4655 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #4655   +/-   ##
=======================================
  Coverage   73.20%   73.20%           
=======================================
  Files         339      339           
  Lines       13320    13320           
=======================================
  Hits         9751     9751           
  Misses       2958     2958           
  Partials      611      611           

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/cmd.go	64.18% <0.00%> (ø)

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[briandealwis]

Odd‚ the status is out of sync: although the pull request is reported as being Queued the actual Travis build succeeded.