Rely on default files list for hex package (#266)

The initial goal for this change was to incorporate the CHANGELOG into
the hex package so that it would be available in the diff tool on hex.pm
or the command line (i.e. `mix hex.package diff bandit 1.1.0..1.1.1`).

Per discussion in #265 we'll accomplish the above by
removing the `files` configuration and relying on the default list of
files provided by hex (see the linked issue for details).

This change will also have the effect of removing test files from future
releases.

Resolves #265

mix.exs

@@ -14,7 +14,6 @@ defmodule Bandit.MixProject do
       description: "A pure-Elixir HTTP server built for Plug & WebSock apps",
       source_url: "https://github.com/mtrudel/bandit",
       package: [
-        files: ["lib", "test", "mix.exs", "README*", "LICENSE*"],
         maintainers: ["Mat Trudel"],
         licenses: ["MIT"],
         links: %{"GitHub" => "https://github.com/mtrudel/bandit"}