npm install isn't run on updaterefs

[AlMcKinlay]

If I just run "guv stop app && guv updaterefs app && guv start app" it updates to the newest commit on the current branch, but it doesn't run npm install, so I have to run it manually.

It still runs npm install when changing branches, or if the branch is force pushed (rebased or similar) but if it's just a fast-forward, it never seems to do that.

Version 3.5.17 Node 6.2.2 Npm 3.10.2