Ensure experiements are a Hash when configuring

Hi, I noticed that if you were to run the following it would raise a

`NoMethodError undefined method 'keys' for false:FalseClass`

if `split-experiments.yml` was empty (`YAML.load_file()` returns `false` in the event of an empty file)

```ruby
Split.configure do |config|
  config.experiments = YAML.load_file("config/split-experiments.yml")
end
```

So, I added some type checking in `Split::Configuration` to short circuit before you even get to that point.

Ash.