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">`
`...`
`</execution>`
`</executions>`
`</result>`

But the `rootXpath()` to get the root xpath for parsing result return an
empty string 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)