Align npm dependencies logic with legacy behavior

[svennam92]

Spawned from issue #20 and #22

1. Turn caching off if node_modules is pushed with application
2. Install new modules if node_modules is pushed with the application, not only rebuild the existing ones.

[cfdreddbot]

Hey svennam92!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you've already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. You can view the current status of your issue at: https://www.pivotaltracker.com/story/show/94525540.

[flavorjones]

Hi @svennam92,

As discussed in #20, we're going to pull upstream changes from Heroku to address #20 and #22. To that end, I'm going to close this PR.

Thanks so much for letting us know we're out of date, and for putting in the effort to describe and fix this behavior. I really appreciate it!

Cheers,
-mike