Extract nif_versions to local variable

Just for maintenance reasons, to make it easier to change it at some
point.

lib/mjml/native.ex

@@ -15,14 +15,19 @@ defmodule Mjml.Native do
     x86_64-unknown-linux-musl
   )
 
+  nif_versions = ~w(
+    2.15
+    2.16
+  )
+
   use RustlerPrecompiled,
     otp_app: :mjml,
     crate: "mjml_nif",
     base_url: "#{github_url}/releases/download/v#{version}",
     force_build: System.get_env("MJML_BUILD") in ["1", "true"],
     version: version,
     targets: targets,
-    nif_versions: ~w(2.15 2.16)
+    nif_versions: nif_versions
 
   def to_html(_mjml, _render_options), do: error()
   defp error(), do: :erlang.nif_error(:nif_not_loaded)