Update Slime Depedency (#19)

Update Slime to 0.13.0 to get multi-line statements.
https://github.com/slime-lang/slime/releases

Author: rubas

mix.exs

@@ -23,7 +23,7 @@ defmodule PhoenixSlime.Mixfile do
     [{:phoenix, "~> 1.1 or ~> 1.2-rc"},
      {:phoenix_html, "~> 2.3"},
      {:cowboy, "~> 1.0"},
-     {:slime, "~> 0.12.2"}]
+     {:slime, "~> 0.13.0"}]
   end
 
   defp package do