Set default value for PhantomJS `Service.process` reference

In the PhantomJS `Service` class, the `process` attribute can be undefined if something goes wrong. In that case, the destructor (`__del__`) raises an `AttributeError`. (That actually happened to me, so it's not just theoretical.) This PR should fix that.

I signed the CLA.

PS: I assume the original author knew about that, but a `__del__` method prevents the garbage collector from cleaning up the memory, if it's not done manually. Also, it is not guaranteed that `__del__` is called: http://stackoverflow.com/a/1481512/284318