Allow stop to fail

I've been bitten by this as well: https://github.com/pat/thinking-sphinx/issues/718 

The reason why stop is getting frozen is because of the `return` inside `ensure` clause. Such construct will stop exception propagation (in this case thrown by `check_for_configuration_file`). 

Since the code in hand doesn't need any specific exception handling I think `ensure` can be removed and just leave the `!running?` value to be returned.