Add nvmrc to droplet and fix failing unit tests

[jmkulwik]

This enforces node v8.15.0 Additionally, this fixes two unit tests that were failing due to commit 07865f25a1f0af430f12e217349d96b8a80190a1

[davidsbailey]

I would suggest also adding a section like this to package.json: https://github.com/code-dot-org/code-dot-org/blob/bf0500cb5c55c8df6c08f502d0a9aa31c805d36a/apps/package.json#L6-L9