Fix for failing extension import

[Raymond26]

Was having issues with:

/usr/local/lib/python2.7/dist-packages/crossplane/__init__.py:5: in <module>
    from .ext.lua import LuaBlockPlugin
/usr/local/lib/python2.7/dist-packages/gevent/builtins.py:93: in __import__
    result = _import(*args, **kwargs)
E   ImportError: No module named ext.lua

[aluttik]

Thanks for pointing this out, but this isn't the best solution. The problem is in setup.py. I'll handle it and contact you when I merge the PR.