fix propcheck and declare xmerl as an extra app required to be turned on

mix.exs

@@ -19,19 +19,18 @@ defmodule Elixlsx.Mixfile do
   end
 
   def application do
-    [ applications: [:xmerl, :propcheck] ]
+    [extra_applications: [:xmerl]]
   end
 
   defp deps do
     [
-      {:credo, "~> 1.6", only: [:dev, :test]},
+      {:credo, "~> 1.6", only: [:dev, :test], runtime: false},
       {:propcheck, "~> 1.4", only: [:dev, :test]},
       {:ex_doc, ">= 0.0.0", only: [:dev], runtime: false},
-      {:dialyxir, "~> 1.0", only: [:dev], runtime: false},
+      {:dialyxir, "~> 1.0", only: [:dev], runtime: false}
     ]
   end
 
-
   defp docs do
     [
       extras: ["CHANGELOG.md", "README.md"],