Cast `timeout` param of SLAssertTrueWithTimeout to NSTimeInterval to avoid warning

I noticed that if you passed just `3` to `SLAssertTrueWithTimeout` that you would get a warning about it not being a double. Since code like `[NSDate dateWithTimeIntervalSinceNow:3]` doesn't require an explicit 3.0, it seems reasonable to not require it for the `SLAssertTrueWithTimeout` macro.