Update Node.js version warning message

[laurenzlong]

Fixes firebase/firebase-functions#292

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[laurenzlong]

I've decided to not include the minor Node version in the message, since sometimes GCF auto-upgrades functions to a new Node version if the existing one has been found to have vulnerabilities.

[CodingDoug]

Could we add a link to the docs that discusses changing that targeted node runtime? The message currently states that node 6 and node 8 are supported, but the dev is given no guidance on how to make a choice, or which is the default.

https://firebase.google.com/docs/functions/manage-functions#set_nodejs_version

[laurenzlong]

Since this warning message isn't just for Firebase, we would also need instructions for gcloud users. I actually wasn't able to find anything on how to configure runtime with gcloud, https://cloud.google.com/sdk/gcloud/reference/functions/deploy does not have any flags for runtime. Perhaps it is not yet updated. Jason, do you have any idea on where to find documentation on selecting runtime? And do you think it would be beneficial to to include links in this warning message?

[jmdobry]

@ace-n Do you know?

[laurenzlong]

Thanks for approving! I don't have authorization to merge the pull request, can you do it if you're happy with it? And perhaps we can do a follow up PR to link to reference pages if one also exists for gcloud?

[gligoran]

Should this be change as well: https://github.com/GoogleCloudPlatform/cloud-functions-emulator/blob/master/bin/emulator#L25-L27?

[Shyam-Chen]

Need to modify the engine used. (#281)

[dungahk]

@gligoran I think that should have been updated, just like the README (https://github.com/GoogleCloudPlatform/cloud-functions-emulator/blob/master/README.md#note)

[grant]

Note: This change has been published.