prepare for v0.6.0 (#318)

Author: polvalente

mix.exs

@@ -1,7 +1,7 @@
 defmodule GRPC.Mixfile do
   use Mix.Project
 
-  @version "0.5.0"
+  @version "0.6.0"
 
   def project do
     [
@@ -53,7 +53,7 @@ defmodule GRPC.Mixfile do
 
   defp package do
     %{
-      maintainers: ["Bing Han"],
+      maintainers: ["Bing Han", "Paulo Valente"],
       licenses: ["Apache 2"],
       links: %{"GitHub" => "https://github.com/elixir-grpc/grpc"},
       files: ~w(mix.exs README.md lib src config LICENSE .formatter.exs)