Upgrade fiber to support node@12

[renfeng]

See laverdet/node-fibers#409

[sodatea]

Thanks for this PR. But I don't think this change makes any difference.
And since we may need to test on Node.js v8, it's better to continue allowing fibers@3 as a devDependency.

[renfeng]

It breaks the build process for projects that depend on it! Can you read this, and reopen it? Thanks

[sodatea]

Could you provide a reproduction?
I don't think it will break any build.

1. It's in devDependencies, won't be installed for those who depend on it.
2. The range >= 3.1.1 <5.0.0 does resolve to 4.0.1 in environments that support fibers@4.x. See the lockfile:

   vue-cli/yarn.lock

   Lines 7018 to 7023 in ab97f31

   	"fibers@>= 3.1.1 <5.0.0":
   	version "4.0.1"
   	resolved "https://registry.yarnpkg.com/fibers/-/fibers-4.0.1.tgz#34fe8a8d010db7f9096adb2558a45bbc1d03fe45"
   	integrity sha512-H79EJn7DMWXk48ygmC82bMP8KNcFBZF1CPfwBpYF6cO85hGWoIrlu7eyX9ayxfjP9Nsl0JXxdI6fpYU4DWVw2w==
   	dependencies:
   	detect-libc "^1.0.3"

   Your PR won't make any difference here.