Fix bug causing KafkaProducer to double-compress message batches

[dpkp]

This addresses producer-side bug from #718 .

[zackdever]

Looks like lz4tools>=1.2.1 is broken for python 2.6. Could try using lz4tools==1.2.0 just for python2.6.

$ python --version
Python 2.6.9
$ pip freeze | grep lz4
DEPRECATION: Python 2.6 is no longer supported by the Python core team, please upgrade your Python. A future version of pip will drop support for Python 2.6
lz4tools==1.2.1
$ python -c "import lz4f"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: dlopen(/Users/zdever/.virtualenvs/tmp26/lib/python2.6/site-packages/lz4f.so, 2): Symbol not found: _PyCapsule_GetPointer
  Referenced from: /Users/zdever/.virtualenvs/tmp26/lib/python2.6/site-packages/lz4f.so
  Expected in: flat namespace
 in /Users/zdever/.virtualenvs/tmp26/lib/python2.6/site-packages/lz4f.so
$ pip freeze | grep lz4
DEPRECATION: Python 2.6 is no longer supported by the Python core team, please upgrade your Python. A future version of pip will drop support for Python 2.6
lz4tools==1.2.0
$ python -c "import lz4f"
$

tox -e py26 passes with the pinned lz4tools==1.2.0 in tox.ini