Investigate automated recipe testing for pull requests

[dogonthehorizon]

Not a high priority feature, but it would be nice to have a macOS executor in Travis that pulls down and installs packages in a given PR (brew is available on the executor, last time I checked).

It'd probably be a little more involved than cloning and installing everything, but some shell-fu could lead to diffing the current PR and only installing the resulting recipes.

Since all good recipes should be implementing their own test block we shouldn't need to do anything other than install the recipe and verify success.