Fix getExecutions not working with api v11

Results return by an API call when calling getExecutions(ExecutionQuery
query, Long max, Long offset) in the RundeckClient class is still the
same in v11 api:
<result success="true" apiversion="11">
<executions count="10" total="20" offset="0" max="10">
<execution id="251" href="http://localhost:4440/execution/follow/251"
status="failed" project="test">
...

But the rootXpath() to get the root wpath for parsing result return an
empty root when using v11 api or upper. For this call i think the result
root is needed (tested with Rundeck 2.1.2 and v11 api)