Fix Python 2.6

Hi György,

thank you for this small but very useful plugin!

I've just stumbled upon a problem. Python 2.6 doesn't understand patterns like `Pattern "{}" not found in "{}"'`, it needs either named or numbered fields. Currently a failed assertion about an error wrapped in `raises_regexp()` triggers `ValueError: zero length field name in format` under py26.

I'm not a big fan of Python 2.6 and I stick to Py3 whenever possible, but I see no reason to drop support for the older Python, for instance, in my [Monk](http://github.com/neithere/monk) library. So it would be great if pytest_raisesregexp could be also tolerant to the old and senile Python.

Thanks!

Andy