Bump Elixir to 1.2 and update Travis CI configuration.

Author: jkakar

.travis.yml

@@ -1,3 +1,5 @@
 language: elixir
+elixir:
+  - 1.2.3
 otp_release:
-  - 17.4
+  - 18.2.1

mix.exs

@@ -9,7 +9,7 @@ defmodule AWS.Mixfile do
      name: "aws-elixir",
      source_url: "https://github.com/jkakar/aws-elixir",
      homepage_url: "http://github.com/jkakar/aws-elixir",
-     elixir: "~> 1.0",
+     elixir: "~> 1.2",
      build_embedded: Mix.env == :prod,
      start_permanent: Mix.env == :prod,
      deps: deps]