Doesn't work against new tox-core

```
Python 2.7.8 (default, Jul 30 2014, 08:21:27) 
[GCC 4.9.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from tox import Tox
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: /usr/lib64/python2.7/site-packages/tox.so: undefined symbol: tox_set_sends_receipts
>>> 
```
@irungentoo/ProjectTox-Core b1a15b197f961aa79ac6e900c6048d6a77f4dc40
PyTox e8bf16b55f4941d6d237a5e7fe464779ea6b1e4e