cant run mapreduce test script on python 3 (missing StringIO)

#129

hey tried running python mapreduce/test_mr_s3_log_parser.py on python 3 and hit this error:

ModuleNotFoundError: No module named 'StringIO'

python 3 moved StringIO into io.StringIO. made a small pr #129 with a try except block so it runs fine on python 3 too.