Make Travis smarter

[FenrirWolf]

Right now our CI process involves downloading a fresh copy of both Rust and devkitARM every time it runs. In fact it actually does all that twice per run since we're testing both against a pinned nightly and the latest nightly.

This is kind of inefficient, plus the folks behind devkitARM don't really like it when automated tools do that kind of thing. Because of that they do provide a Docker container that's all set up for 3DS dev at https://hub.docker.com/r/devkitpro/devkitarm/, so we should find a way to integrate that into our compile tests if possible.

Testing against both the current and latest nightlies might be unnecessary as well, and maybe we should just test whichever one we're currently pinned to.