Fixed dep warnings for List.concat

Was getting dep warning:
```
List.concat/1 is deprecated, please use Enum.concat/1 instead
    /Users/ortuna/Desktop/code/opensource/elixir/lib/elixir/lib/list.ex:17: List.concat/1
    lib/ecto/query/validator.ex:332: Ecto.Query.Validator.group_by_entities/2
    lib/ecto/query/validator.ex:34: Ecto.Query.Validator.validate/3
    /Users/ortuna/Desktop/code/opensource/elixir/lib/ex_unit/lib/ex_unit/assertions.ex:342: ExUnit.Assertions.assert_raise/2
    /Users/ortuna/Desktop/code/opensource/elixir/lib/ex_unit/lib/ex_unit/assertions.ex:316: ExUnit.Assertions.assert_raise/3
    test/ecto/query/validator_test.exs:47: Ecto.Query.ValidatorTest."test where expression must be boolean"/1
    /Users/ortuna/Desktop/code/opensource/elixir/lib/ex_unit/lib/ex_unit/runner.ex:138: ExUnit.Runner."-run_test/3-fun-0-"/3
```