more permissive regex for hdfs stat cli response

I am using Hortonworks' HDP distribution of Hadoop, and needed to make this small patch to luigi in order to have it run tasks with an HDFS output.

```
[root@hadoop1 ~]# yum list installed | grep hadoop
hadoop.x86_64                      1.2.0.1.3.3.0-58.el5               @Updates-HDP-1.x
```

Sample existing stat response:
```
[root@hadoop1 ~]# hadoop fs -stat /data
2013-11-21 21:10:54
```

Sample non-existing stat response:
```
[root@hadoop1 ~]# hadoop fs -stat /path/that/doesnt/exist
stat: cannot stat `/path/that/doesnt/exist': No such file or directory
```