Update dependency png to ~> 0.2

mix.exs

@@ -41,7 +41,7 @@ defmodule Vivid.PNG.Mixfile do
       {:ex_doc, ">= 0.0.0", only: ~w[dev test]a},
       {:credo, "~> 1.1", only: ~w[dev test]a, runtime: false},
       {:git_ops, "~> 2.3", only: ~w[dev test]a, runtime: false},
-      {:png, "~> 0.1"},
+      {:png, "~> 0.2"},
       {:vivid, "~> 0.4"}
     ]
   end