This will make travis only fetch the top level commit instead of
fetching the top 50 (default). Nox should be able fetch other commits on
an as-needed basis.
This adds a 3rd matrix to be built by Travis. The new matrix "checks"
the NixPkgs evaluation so the other 2 can save their resources for
building. Hopefully, this will lead to less "out of space" errors that
seem to be happening with Travis. Also adds folding.
