python version

Hi, 

Looks like code needs python version2. tried with latest ycsb as well, similar error. 

./bin/ycsb run mongodb-async -s -P workloads/workloada > outputRun.txt
  File "/root/mongo/ycsb-0.5.0/./bin/ycsb", line 157
    except subprocess.CalledProcessError as err:

Thanks

