Only test against Node 9 on master (#922)
Only test against Node 9 on master (#922)

Since it's very unlikely we're ever going to have a Node 9 only CI
failure if Node 8 and Node 10 both pass. This means each PR/branch
only uses 2 jobs rather than 3, out of our 5 concurrent job
allowance - but yet we still have coverage once they are merged to
`master`.

Node 9 will also no longer be supported after the end of June.
edmorley/neutrino-dev,mozilla-neutrino/neutrino-dev,mozilla-neutrino/neutrino-dev,edmorley/neutrino-dev