Simplify S3.open_file with "with" form

lib/asciinema/file_store/s3.ex

@@ -38,8 +38,7 @@ defmodule Asciinema.FileStore.S3 do
   def open_file(path, function \\ nil) do
     response = bucket() |> S3.get_object(base_path() <> path) |> make_request
 
-    case response do
-      {:ok, %{headers: headers, body: body}} ->
+    with {:ok, %{headers: headers, body: body}} <- response do
       body =
         case List.keyfind(headers, "Content-Encoding", 0) do
           {"Content-Encoding", "gzip"} -> :zlib.gunzip(body)
@@ -51,8 +50,6 @@ defmodule Asciinema.FileStore.S3 do
       else
         File.open(body, [:ram, :binary, :read])
       end
-      {:error, reason} ->
-        {:error, reason}
     end
   end