added configuration for gigalixir

elixir_buildpack.config

@@ -0,0 +1,3 @@
+elixir_version=1.10.2
+erlang_version=22.2
+release=true

mix.exs

@@ -5,7 +5,7 @@ defmodule PodcastFeed.MixProject do
     [
       app: :podcast_feed,
       version: "0.1.0",
-      elixir: "~> 1.5",
+      elixir: "~> 1.7",
       elixirc_paths: elixirc_paths(Mix.env()),
       compilers: [:phoenix, :gettext] ++ Mix.compilers(),
       start_permanent: Mix.env() == :prod,