[CI] Consider adjusting Travis' caching policy

[clintfred]

I noticed that our cache sizes are getting pretty big.

According to https://levans.fr/rust_travis_cache.html, Travis is caching all old versions of our dependencies, so that number will only get bigger over time. Since Travis has to fetch and unpack that cache each build, it may not be saving more time than it's costing -- at least at some point it won't.

We should try a PR with the caching policy adjusted and see how things perform.