Update Tesla to 1.3

This version includes a couple of fixes, adds Gun and Mint adapters and
removes 0.x -> 1.0 config migrator, which for some reason fails under
certain conditions. I had to set `override: true` because Quack pins
Tesla to `1.2.0`, but I have looked through the source code and verified
that updating Tesla doesn't break anything there.

mix.exs

@@ -113,7 +113,7 @@ defmodule Pleroma.Mixfile do
       {:calendar, "~> 0.17.4"},
       {:cachex, "~> 3.0.2"},
       {:poison, "~> 3.0", override: true},
-      {:tesla, "~> 1.2"},
+      {:tesla, "~> 1.3", override: true},
       {:jason, "~> 1.0"},
       {:mogrify, "~> 0.6.1"},
       {:ex_aws, "~> 2.1"},