fixes StringIO unicode handling in python 2.7

This patches fixes unicode handling in StringIO for python 2.7 and below. The patch causes python 2.7.x and earlier to use StringIO.StringIO instead of io.StringIO.

Test script dumping received packets using collectd_network.py generates exception under python 2.7. This is due to io.StringIO's handling of unicode.

```python
Traceback (most recent call last):
  File "test.py", line 10, in <module>
    print x
  File "/home/else/code/collectd-5.3.0/contrib/collectd_network.py", line 219, in __str__
    return "%s %s" % (Data.__str__(self), list.__str__(self))
  File "/home/else/code/collectd-5.3.0/contrib/collectd_network.py", line 181, in __str__
    return "[%i] %s" % (self.time, self.source)
  File "/home/else/code/collectd-5.3.0/contrib/collectd_network.py", line 165, in source
    buf.write(str(self.host))
TypeError: unicode argument expected, got 'str'
```

Test script (run it and point collectd at it):
```python
from collectd_network import *
reader = Reader()
while True:
    interpreted = reader.interpret()
    for x in interpreted:
        print x
    print '------------------------------------------'
```