Add node_modules cache to travis

[weswigham]

This could speed up CI builds a little bit.

See https://docs.travis-ci.com/user/caching/ for more information.

[vladima]

is it possible to explicitly force cache invalidation in cases if our dependencies really should be updated?

[kitsonk]

Those authorised on the Travis account can clear the cache.

[weswigham]

It looks like this cuts about a minute and a half off our travis builds. I think this is good.

[weswigham]

@vladima

We should only have to run npm update to make sure our deps are updated in the cache.... however since we transitively depend on ourselves, npm update refuses to run. This can be worked around by changing the name in our package.json before running npm update, then changing it back... but this still has a downside: only our top-level dependencies get checked. If we add the argument --depth 999, theoretically we could catch everything, but when we use depth at all we hit this npm bug.

If the partial solution is okay (despite the circular dependency hack) I can add it to this PR.

[weswigham]

@vladima Look like my new strategy for cache busting works! (Which is "don't cache packages npm has trouble with", like typescript or tslint). This shaves > 1 minutes off of our builds - should we merge with master now?

[sandersn]

👍