Bump version to 2.4.0

Bumps the version to 2.4.0. The only real change here is the ability to
use ExMachina.Ecto without a repo (since Ecto 3 split ecto and ecto_sql
into separate packages).

CHANGELOG.md

@@ -7,6 +7,15 @@ complete changelog, see the git history for each version via the version links.
 
 [hex package page]: https://hex.pm/packages/ex_machina
 
+## [2.4.0]
+
+### Added
+
+- Allow ExMachina.Ecto to be used without :repo option ([#370])
+
+[2.4.0]: https://github.com/thoughtbot/ex_machina/compare/v2.3.0...v2.4.0
+[#370]: https://github.com/thoughtbot/ex_machina/pull/370
+
 ## [2.3.0]
 
 ### Added

README.md

@@ -20,7 +20,7 @@ In `mix.exs`, add the ExMachina dependency:
 def deps do
   # Get the latest from hex.pm. Works with Ecto 3.0
   [
-    {:ex_machina, "~> 2.3"},
+    {:ex_machina, "~> 2.4"},
   ]
 end
 ```

mix.exs

@@ -2,7 +2,7 @@ defmodule ExMachina.Mixfile do
   use Mix.Project
 
   @project_url "https://github.com/thoughtbot/ex_machina"
-  @version "2.3.0"
+  @version "2.4.0"
 
   def project() do
     [