Upgrade Elixir and remove .buildpacks

.buildpacks

@@ -1,4 +0,0 @@
-https://github.com/gigalixir/gigalixir-buildpack-clean-cache.git
-https://github.com/HashNuke/heroku-buildpack-elixir
-https://github.com/gjaldon/heroku-buildpack-phoenix-static
-https://github.com/gigalixir/gigalixir-buildpack-distillery.git

.tool-versions

@@ -1,4 +1,4 @@
-elixir 1.11.3
+elixir 1.11.4
 python 2.7.14
 nodejs 10.16.0
 erlang 23.2.3

Dockerfile

@@ -0,0 +1 @@
+FROM elixir:1.11.4

mix.exs

@@ -5,7 +5,7 @@ defmodule Chess.Mixfile do
     [
       app: :chess,
       version: "0.2.0",
-      elixir: "~> 1.11.3",
+      elixir: "~> 1.11.4",
       elixirc_paths: elixirc_paths(Mix.env()),
       compilers: [:phoenix, :gettext] ++ Mix.compilers(),
       build_embedded: Mix.env() == :prod,