Cache node and Yarn installation

[jonas]

From https://github.com/olafurpg/metadoc/pull/4#issuecomment-310866379:
It seems npm and yarn is installed on every ci run and node_modules is not cached. It would be nice to cache those too, but I don't know what's the best way to achieve that. The yarn.lock and node_modules are located in nested directories.

[olafurpg]

We could embed an sbt-launcher.jar in the repo and configure .travis.yml to be a js project. It should be possible to cache the nested node_modules directories, just by hard-coding them.

[gabro]

Just throwing there a random idea, but due to the cross-breed nature of the project it would probably be easier to use Drone instead of Travis, crafting a custom Docker image with both scala and js.

However that would require hosting an installation of drone, but there may be one in place already for the Scalacenter, right @olafurpg?

[olafurpg]

I prefer to use Travis because it's a lot simpler to setup caching. I was the one pushing for Drone at the SC originally, but at this point I've spent so much time on it that I prefer Travis.

[olafurpg]

After #101 the npm download size is significantly smaller, CI went from 10 minutes to 5 minutes. Would still be nice to cache node_modules.