Use Logger.warning instead of Logger.warn (#35)

* Bump minimal Elixir to 1.11

So we can use Logger.warning instead of Logger.warn

* Use Logger.warning instead of Logger.warn

lib/main_proxy.ex

@@ -62,7 +62,7 @@ defmodule MainProxy do
         backends
 
       :error ->
-        Logger.warn(
+        Logger.warning(
           "No backends specified. Either configure :main_proxy, :backends or define a " <>
             "`backend/0` function in your `Proxy` module."
         )

mix.exs

@@ -8,7 +8,7 @@ defmodule MainProxy.MixProject do
       description:
         "Proxies requests to multiple apps. Useful for Gigalixir or Heroku deployment when just one web port is exposed. Works with phoenix endpoints, plugs, and websockets.",
       package: package(),
-      elixir: "~> 1.9",
+      elixir: "~> 1.11",
       elixirc_paths: elixirc_paths(Mix.env()),
       start_permanent: Mix.env() == :prod,
       deps: deps(),