force wx version 2.8 for compatibility

This is required on systems that have wxgtk-3.0 installed, otherwise it throws a

    /usr/lib/python2.7/site-packages/wx-3.0-gtk2/wx/_core.py:16629: UserWarning: wxPython/wxWidgets release number mismatch
    warnings.warn("wxPython/wxWidgets release number mismatch")

warnings and also breaks later on while starting up.