Fix endpoint getting fixed at compile-time

lib/binance/rest/http_client.ex

@@ -1,7 +1,9 @@
 defmodule Binance.Rest.HTTPClient do
-  @endpoint Application.get_env(:binance, :end_point, "https://api.binance.com")
+  defp endpoint() do
+    Application.get_env(:binance, :end_point, "https://api.binance.com")
+  end
 
   defp prepare_request(url, params, secret_key, api_key) do
     case validate_credentials(secret_key, api_key) do
       {:error, _} = error ->
         error
@@ -32,7 +34,7 @@
   end
 
   defp request_binance(url, body, method) do
-    url = URI.parse("#{@endpoint}#{url}")
+    url = URI.parse("#{endpoint()}#{url}")
 
     encoded_url =
       if body != "" do
@@ -111,7 +113,7 @@
     request_binance(url, argument_string, method)
   end
 
   defp validate_credentials(nil, nil),
     do: {:error, {:config_missing, "Secret and API key missing"}}
 
   defp validate_credentials(nil, _api_key),
@@ -123,7 +125,7 @@
   defp validate_credentials(_secret_key, _api_key),
     do: :ok
 
   defp parse_response({:ok, response}) do
     response.body
     |> Poison.decode()
     |> parse_response_body
@@ -133,7 +135,7 @@
     {:error, {:http_error, err}}
   end
 
   defp parse_response_body({:ok, data}) do
     case data do
       %{"code" => _c, "msg" => _m} = error -> {:error, error}
       _ -> {:ok, data}