Fixed JSON serialisation of MultiInstance.times attribute (present in wo...

XMLSerializer allows MultiInstance.times to be an Attrib causing type errors when re-serializing to JSON:
```
from SpiffWorkflow.storage import *
x_ser = XmlSerializer()
spec = x_ser.deserialize_workflow_spec(open('tests/SpiffWorkflow/data/spiff/workflow1.xml').read())
j_ser = JSONSerializer()
j_data = j_ser.serialize_workflow_spec(spec)
jspec = j_ser.deserialize_workflow_spec(j_data)
...
  File "./SpiffWorkflow/storage/JSONSerializer.py", line 40, in default
    raise TypeError('%r is not JSON serializable' % obj)
TypeError: <SpiffWorkflow.operators.Attrib object at 0x10b024410> is not JSON serializable
```
Im not sure if this is a specification inconsistency. If it is then perhaps the appropriate fix is in XMLSerializer instead.