use native travis support for yarn

[trevj]

This change is 

[trevj]

Based on:
https://blog.travis-ci.com/2016-11-21-travis-ci-now-supports-yarn

Thanks for the pointer, @jab!

[jab]

Nice @trevj! 👍 👍 👍

[jab]

Would we not want to cache node_modules too, as mentioned in the blog post?

cache:
  yarn: true
  directories:
    - node_modules

[fortuna]

Thanks for the change!

@jab, I'm not sure it makes sense to cache the node_modules, as they should differ from different runs. Yarn will recreate it from its own cache.

[jab]

@fortuna Won't it still help for the requirements that stay the same between runs? That's how it works with a pip (Python installer) directory I have Travis set up to cache in one of my projects, and is also what that blog post @trevj linked to above demonstrates doing, no?

[trevj]

I just tried the blog post way with node_modules explicitly called out but honestly it's hard to see a difference - I suspect @fortuna is right to say that since yarn does its own caching and is fast that caching node_modules explicitly probably doesn't make much sense.

I think I'll revert to the simple cache: yarn.

[jab]

Ok, looks great! 👍