We are seeing more and more people installing on Node v7 so it's important to ensure `npm install` will pass correctly and our (limited but growing) test suite as well.
OSX builds have been nothing but a pain on Travis CI: they fail with no good reason, they stay pending forever, etc.
As far as I can tell, I can't remember one valid build they failed and we legitimately discovered a bug. Dev env on OSX is very close to Linux so it's good enough to have it here.
This allows for a more meaningful build: if a newer version of a sub-package breaks,
builds would still pass as it uses the cached version. This uses a cache for downloaded packages instead.
I am expecting this to slow down a little bit the builds (but we are OK overall)
but be more accurate in practice.
See https://docs.npmjs.com/cli/cache#configuration and https://docs.npmjs.com/files/folders#node-modules.
