Prepare the official 1.3 release (#151)

Author: jparise

README.md

@@ -10,7 +10,7 @@ to the default reporter for your environment.
 Add the following to your dependencies in mix.exs:
 
 ```elixir
-{:elixometer, "~> 1.2"}
+{:elixometer, "~> 1.3"}
 ```
 
 Or to track the master development branch:

mix.exs

@@ -11,7 +11,7 @@ defmodule Elixometer.Mixfile do
   def project do
     [
       app: :elixometer,
-      version: "1.3.0-rc1",
+      version: "1.3.0",
       elixir: ">= 1.5.0",
       description: @description,
       source_url: @project_url,