Fix version numbers

[SerpentChris]

The current setup.py sets the version number to 0.1.23, whereas the current PyPI version of this package is listed as version 0.1.27. Why is this a problem? Because the PyPI version does not contain the endian fix that the github version has, which means that anyone trying to install this via PyPI on a big endian system is gonna have two problems: The first is that the PyPI package fails to build. This is easily worked around by cloning from github and installing with python setup.py install. The second problem is that other ethereum python packages (like py-evm) assume that the latest version of pyethash is the best and they want a version >=0.1.27. To fix that you have to clone py-evm and then edit the setup.py to force the version from the ethash repo, 0.1.23.

I think this can be fixed by bumping the version number here to 0.1.28, and then updating the pypi package.