River now requires Elixir v1.5+

This is because we're using the dynamic supervision support of the
:simple_one_for_one strategy, to which we pass child specs generated via
Supervisor.child_spec/2. We could probably route around this by
generating the child specs directly, though... hrm

.travis.yml

@@ -1,14 +1,8 @@
 language: elixir
 elixir:
-  - 1.3
-  - 1.4
   - 1.5
 otp_release:
   - 17.5
   - 18.3
   - 19.0
   - 20.0
-matrix:
-  exclude:
-  - elixir: 1.3
-    otp_release: 20.0

mix.exs

@@ -4,7 +4,7 @@ defmodule River.Mixfile do
   def project do
     [app: :river,
      version: "0.0.4",
-     elixir: "~> 1.3",
+     elixir: "~> 1.5",
      description: description(),
      package: package(),
      build_embedded: Mix.env == :prod,