Cache composer assets on Travis

Now we are on the container-based infrastructure we can cache assets between builds - this will make the build a lot quicker.
Nah, not at all. dists are smaller too normally, that is if people have setup their .gitattributes correctly. They were just a bad idea because they'd bust the low api rate limit, which is selfish, because other people using the vm later might have a genuine need to use the api.