Fix UninitializedFieldError when using RedisScalatraBroadcasterConfig

Forgot that I hadn't pushed this particular commit when issuing the pull request.

Overriding the trait `def`s using `val` in an extending class doesn't work, so I
needed to change it to `def` in the `RedisScalatraBroadcasterConfig` class.
Merge pull request #365 from lloydmeta/fix/RedisBroadcastConfig

Fix UninitializedFieldError when using RedisScalatraBroadcasterConfig