Ensure non `/assets/` files can be served.

Previously, any files in `public/` (outside of `public/assets/`) would
not be served up as it was assumed to match history API requests and
redirect to `/index.html`.

This commit fixes that, by serving up any files that exist in the built
output directory before falling back to checking for history API support
(and subsequently proxying).
Merge pull request #1549 from rwjblue/fix-public-dir

Ensure non `/assets/` files can be served.