use six.StringIO for compatibility with io.StringIO in python3

The newer version of six (1.4.1) provides six.StringIO which
is a fake file object for textual data. It's an alias for cStringIO.StringIO,
in python2 and io.StringIO in Python3.

Change-Id: If1ccfca128385f976938f0f0c74573687faad9ac
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "use six.StringIO for compatibility with io.StringIO in python3"