Fix npm prerelease

[fcollonval]

Fixes #730

Set tag for npm publication according to the pre-release status of the GitHub release.

[fcollonval]

@meeseeksdev backport to 0.11.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 0.11.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 d52688c8935108dc9520a9ddd8c4a3eaa5c0a168

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #732: Fix npm prerelease'

4. Push to a named branch :

git push YOURFORK 0.11.x:auto-backport-of-pr-732-on-0.11.x

5. Create a PR against branch 0.11.x, I would have named this PR:

"Backport PR #732 on branch 0.11.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.